Monotremes are the oddballs of the mammal family, the echidnas and especially the platypus are absolutely absurd creatures with their bizarre anatomical features and mishmash of reptilian and mammalian traits. Of particular note is that they lay eggs and run a much lower core body temperature than other mammals.
These traits at first appeared to be nothing more than a a curiosity and sign of their primitiveness, but I pondered the advantages such a “toolkit” might bring. I came to the conclusion that monotremes actually have features that would be extremely beneficial for growing to massive sizes that placental mammals cannot reach.
Although the idea seems absurd on its face, we can examine what constrains the size of placental mammals and examine traits of what we know to be the largest land animals to ever exist; the sauropod dinosaurs.
Placental mammal size is quite limited compared to the dinosaurs for three primary reasons; metabolism, reproduction, and biomechanics. Placental mammals have to eat a lot of food to keep their fast metabolisms going, while reptiles and monotremes need to eat a lot less. Pregnancy in placental mammals is arduous on the mother and simply put the larger the baby the longer and more costly the pregnancy. Egg laying gets around this as it takes far less energy and maternal commitment. Dinosaurs also had a different bone structure and air sacs that allowed them to grow larger, so both placentals and monotremes lose out on those.
I’m not expecting the monotremes to reach sauropod size, but given that they have the metabolism and reproductive method suited for large sizes of expect them to at least have be reaching larger sizes than the placentals.
But my conjecture isn’t borne out by evolutionary history, as the largest monotreme was about sheep sized. So what prevents the monotremes from getting big besides getting pushed to the very fringe by the other two mammal families? What would it take for gigantic monotremes to evolve? Would being isolated on a continent free of other mammals suffice? Or is there something else?