That's why mammals can't breathe at high altitudes that birds can, but I'm not sure if that affects the body plan much in terms of size. The largest birds are smaller than the largest mammals on land or at sea. Then again, lower oxygen levels compared to the past seems to be a limitation for insect sizes too (who have an even less efficient respiratory system).
I also don't think it's the warmbloodedness. There are giant mammals in general after all.
Perhaps it is because bats form large, dense colonies? There is only so many resources available in any given ecological niche, so then for any species that fills a niche one would expect those resources to be divided either among many small individuals or a few large ones. Bat evolution chose the "big colony" route, which I assume favors smaller individuals.
> The largest birds are smaller than the largest mammals on land or at sea
With all my respect to you theory I think comparing size of animals should not ignore the medium they moved in: water, land or air. Weight is (loosely but still) related to size. It’s probably not a coincidence the largest mammals lives on water where they need less energy to supper their weight, and it’s not a coïncidents the largest mammals on earth are way bigger that bats.
The biggest bats are ~1.7m which is not so far from biggest albatros (3.7m).
Also consider the biggest bird (Ostriches) can’t fly. Now I’m trying to picture a swimming gigantic bird.
Well, fair. But birds are warmblooded too so that doesn't change much there, and on top of that the difference in requiring bigger lungs for the same amount of oxygen extraction would exactly add much weight per volume, so to speak
I also don't think it's the warmbloodedness. There are giant mammals in general after all.
Perhaps it is because bats form large, dense colonies? There is only so many resources available in any given ecological niche, so then for any species that fills a niche one would expect those resources to be divided either among many small individuals or a few large ones. Bat evolution chose the "big colony" route, which I assume favors smaller individuals.