And vice versa, Birds are basically Dinosaurs. Hollow boned, warm blooded, and covered in insulating feathers. If the planet was about ten degrees warmer with CO2 levels several times higher than now (in that order, by the way, Mr. Gore) they could still be growing to gargantuan size.
They’d really need a lot more atmospheric pressure to grow that large again. That’s the only thing I think could offset gravity enough to keep the large ones from collapsing under their own weight.