This concept is relatively simple. There are many animals which start mobile, but become sessile later in their life history, all of which are simple and primitive. And I was wondering this: what is the upper limit on animal size? And I realized that in the ocean, that limit is probably simply determined on not being so massive that it kills you. But on land, you would die from being unable to move, find food and water etc., long before you die from suffocating under your own weight. So I realized you could have a mammal (or other complex animal, even an insect would do) that becomes immobile as it grows up, and uses pheromones to attract other lifeforms, like something akin to a carnivorous plant. Could that work? Why or why not? Related issues: Is it possible for mammals to undergo metamorphosis? Why or why not? Could, say an elephant, do this without metamorphosis? By no, do you mean not feasible, or could not evolve but could engineer?
Just because it's sessile doesn't mean it can't defend itself.
I have in mind as a starting point some jellyfish that have algae inside themselves--a symbiosis that in effect produces a photosynthesizing animal.
Lets scale this way, way up. The adult form of the creature is immobile. Rather than a symbiosis with algae it's partner is some sort of plant. The plant is taproot-based, the root extends down through a channel in the host and into the ground. The photosynthetic organs (probably not leaves) grow behind a clear protective layer in the host.
While the body of the creature is fixed it retains parts that can move--specifically, some sort of arm or pseudopod. The creature is hexagonal and has 7 such pseudopods--one on top which can deal with birds.
The young are born mobile. When they're grown enough they leave home and find another creature to mate with--this bond is permanent, they take up residence touching their mate and eventually the touching faces grow together. If a youngster can't find a suitable location next to an existing creature they instead pick a suitable location to live and settle in there--they are now the sessile form that another youngster may choose to mate with.
Creatures that are not on the edge of the collection retain only their upper limb and in practice this will lie unused for great periods of time (the birds know not to land there, there will be almost no need to actually defend itself) and become pretty much just energy production and storage.
Since the creature grows by aggregation rather than straighforward growth there's no upper limit on it's size--it will expand until environmental or terrain limits stop it.
On land the size is not merely determined by the ability to move around for food, but also the skeletal and skin concerns. In water most of the weight of the animal is supported by the buoyancy of water, but on land there is negligible buoyancy offered by air, so all weight must be supported by the animal's own skeleton.
So while giants like blue whale and megalodon appeared time and again in the oceans, land giants were much smaller (in mass) than them. For example, while there did roam diplodocus and saltasaurus and seismosaurus on land, their weights were lesser (comparatively) than the weights of sea creatures of similar size. In other words, you can say that dinosaurs' were more long than fat. There is little such restriction in the water though.
The first hindrance to a static animal is not how it would get food, but how it would stop becoming the food of other animals. Furthermore, animals' circulatory and digestive systems require at least some movement on the part of animals (anything larger than a cat that is). So even if you have an animals which you continue to feed, it will get sick and die if it doesn't move. This is especially true for mammals. While a boa constrictor would sleep away for most of it's life, a mammal would be much more active.
One potential (though mildly disturbing) possibility is the logical extension of a naked mole rat colony. Naked mole rats are eusocial mammals, meaning that the reproductive members of their society and the non-reproductive ones lead different lives (somewhat similar to the way workers and queens in an ant colony are different).
It's not massively hard to imagine a situation in which the females in our new species (let's call it the Couch-potato mole rat) enjoy increased fecundity and lower mobility as they age. Usually a creature losing mobility in old age is just a good way for it to die, but in this eusocial colony setting there is a good reason for the other rats to care for the increasingly sessile Matriarch: namely that she's popping out broods of ratlings like they're going out of style. The logical conclusion of this is a society where the reproductive females find a good burrowing spot, and then all the other rats do the hard work of feeding her, managing her temperature, removing the broods of new rats and defending her. The burrows all around would turn the nearby area into a natural pit trap, and with hundreds of wrinkly little minions hiding in the tunnels, any animal straying there would be certain to have an unpleasant time at the best.
Now: The above answers the question of 'is it possible to have an animal become sessile as it grows up', and gives a potential evolutionary path for it. As for the question of the upper limit for size: That's a beast of a whole different colour. For now I'm going to assume that once a Couch-Potato mole rat becomes sessile it begins to grow until it reaches a nutritional plateau or dies. Mole rats are shown to be resistant to cancers, have extremely long lives for their size and have some very odd metabolic tricks up their wrinkly little sleeves, so that's not outside the realm of possibility.
Looking at the comments of your question I'm assuming that you want these Matriarchs to be visible aboveground, which is a terrible idea as it opens them up to all manner of predation that subterranean life doesn't. It does, however, lend itself well to growth, as the Matriarch no longer has to worry about burrow size or dealing with collapses. For now let's say that when the matriarch breaks ground the non-reproductive males act as warriors, fiercely gnawing at any predator, and hanging onto the hide of their mother to defend against bird predation. Usually this would be a terrible idea, but with the Matriarch popping out replacement soldiers the male mole rats can literally mob predators as they appear, then feed the broken carcasses to their ever expanding mother. If a colony were successful enough then the Matriarch could begin to grow to truly gargantuan sizes, consuming ever more food brought to her by her minions and throwing out even more impressive broods.
Now we start to run into some structural issues. For starters: The mole rat is going to have to remain... accessible to the reproductive males in the colony and to the workers scurrying to remove fresh broods. She also has to be able to eat food brought to her by her minions continuously. This means that her hindquarters can't be covered by the rest of her body, and that the head needs to remain accessible and of such a size that her minions can feed her easily.
This essentially means that the torso is the only bit that can grow: leading to a fat, bloated ball of wrinkly pink flesh. The main structural component in this case is going to be the ribcage, as it's very quickly going to have to bear the brunt of the matriarch's weight. She could lie on her back, but then her reproductive organs would be crushed under their own weight as opposed to hanging down from her spine. Even if she does lie on her back eventually the sheer bulk of muscle needed to breath leads to breathing problems and (as mentioned in your OP) the Matriarch suffocating under her own weight. You can scale up or down as much as you like if we've taken this scenario to this extreme, but I think a sensible size for a fully grown matriarch would be 1-2 meters across, hardly the Behemoths you envisaged.
Though it is one of the more horrifying creature concepts I've thought about in a while. Especially if you cross these rats with Star-nosed moles, and give them a taste for burrowing through flesh...
If you get away from animal life on this planet, evolution might have explored other pathways. The most massive known living organism on Earth is a fungus, measuring about a hundred miles across, growing in the USA prairie. Its fruiting bodies look like insignificant toadstools, but they're all genetically identical and connected underground!
Plants on this planet obtain mobility by having seeds that hitch a lift on (or inside) animals and birds, or blowing on the wind. Elsewhere, you might have plants with autonomous seeds like mayflies or mice, which then take root when they find an appropriate place to bury themselves. If they were voracious predators on other species' small autonomous seeds while they sought a place to take root, you might have a world where animal life never evolved, and sentient forests think long deep thoughts ....
[added later] Another possibility is further evolution of the eusocial insects or equivalents. One mind, very many bodies. I don't know of any insect colonies here on earth that mass more than an elephant or which are anything like as smart as an elephant, but perhaps after we blow ourselves up, it'll be the termites that win in the long term. (It's been written. Charles Stross, "Missile gap", very nightmarish).
Several issues here:
Sessile plants consume a lot less energy than motile animals. So evolutionary pressure for a sessile stage to a life cycle of an otherwise motile creature could arise if the planetary environment underwent a phase in which the energy input available to such motile creatures periodically declined sufficiently to make mobile search for food unattractive energetically. If this were combined with a simultaneous increase in the energy available from photosynthesis then such a lifecycle could be plausible.
Mammals do undergo metamorphosis, they just do so in utero - in the womb. What we typically think of as metamorphosis occurs ex-utero, after birth. Metamorphosis is to my understanding an hormonally triggered/guided process. In theory mammals could undergo it, I think. However they would need to have the required processes and biochemical pathways built into their ontogony from the beginning. If in the sessile phase they can photosynthesise, then where does this capability come from?
For example, I have in mind a creature which is birthed from a sessile egg and plant combination (OK, lets call it an "eggplant"), in which the eggplant uses photosynthesis to provide the main nutrients and biomass for the new organism. Within the eggplant, animal-like processes (e.g. hetrotrophic processes) consume the biomass created via photosynthesis to create the newborn. If we assumed an environmental change that makes most motile heterotropic strategies unattractive, then we may also assume that predators of your lifeform also face similar pressures and either adopt a similar sessile stage, or decrease significantly in numbers.
In order to fulfil the requirement that this life form is a mammal, I shall specify that the eggplant is 'birthed' from an internal space of the mature mammal-phase. Thus the 'newborn' is actually the eggplant. The "metamorphosis" is the emergence of the adult, mobile phase from the eggplant. The cycle then continues.
I'll leave it to you to fill in the details of how the environment works to drive these processes.