Big flightless birds extinct and extant have a similar body habitus - long neck, bulky body, no tail.
Probably it is the lack of tail for balance combined with a need for speed that limits the maximum size of these large flightless birds.
Theropod dinosaurs, the ancestor of birds, walked with their legs
vertical using their tail for balance. As their descendents evolved to
fly, their heavy tails became a liability and shrank to a stub. To
keep their balance, the evolving birds began to crouch, with their
femurs tilted back to keep their knees under their centre of gravity.
They kept this position even after some had ceased to fly, and the
extra strain made their femurs stronger... The dino femurs grew longer
and thicker as body size increased, Chan says. But in birds, there was
a “distinct plateau” between 300 and 500 kilograms, with femur length
constrained by the need to keep the knee near the centre of gravity.
That makes it harder to support a large body, and so it limits their
size, argues Chan.
I wonder also if there is a limit to how strong a hollow femur bone can be. These flightless birds have strong bones, but they are all still hollow. Also the light skeleton reduces the mass of these birds - a giraffe of 5 meters might weigh 1000 kg (weight to height ratio = 2) but a big 3 meter ostrich weights only 150 kg (weight to height = 0.5). Good if you want to be a fast bird, but not if you are trying to achieve a weight target requested on world building stack.
But there is a different type of body a flightless bird can have.
Penguins are unusually upright, with their legs set far back on their bodies. Uniquely among birds their bones are not hollow. Penguin tails, or pygostyles are very robust.
Penguins have a pygostyle, but it is quite different in shape from the
standard avian pygostyle. In penguins, the element is more elongated
and less flattened. Rather than being plate-like, it is almost
triangular in cross-section with a flattened base. Penguins also have
a very different set of tail feathers. Rather than forming a fan,
penguin tail feathers are very stiff and quill-like, and stick out
somewhat like the bristles of a broom. This is especially true of
penguins from the genus Pygoscelis – the Linnean name Pygoscelis
actually translates to “stiff tail”. These penguins are prone to be
caught slouching around, partially propped up on their tail feathers.
It seems that without the necessity of maintaining a “fan” of tail
feathers, penguins have gone ahead and modified their pygostyle to a
shape more suited to supporting themselves on land than steering in
This has the makings of a large land animal: weight stacked vertically over the stable tripod of stocky legs and tail. This is a different route to bipedalism than either an ostrich or a tyrannosaur. This is a thing that can get very big - like the ground sloth Megatherium which could measure 6m and weigh 4000kg.
The giant terror penguin will, however, be a predator in the pattern of a tyrannosaur. Why did the Tyrannosaur need to be so big and fierce? Was it to take down and devour huge formidable prey? No - it was to intimidate smaller, fierce carnivores. A carnivore never wants a fight it might not win - any injury might prove fatal because of consequent inability to hunt and starvation. Wild dogs are tremendously efficient hunters but half of their kills are stolen from them by larger predators.
Like T.Rex (may have been!) the massive terror penguin is a scavenger and kill stealer. No other predators have a chance against it and they leave when it arrives, carrying off what they can. Its bulky body offers no vulnerable spots within reach, and the bulk serves to anchor a long snakelike neck and sharp slashing beak. Because of its diet of carrion, its head will be bald, like a marabou stork or vulture. The long neck also allows it to eat without actually getting down prone on the ground, and the size of the bird means it can eat the entire kill itself.