>>11677971Your surface area increases by the square, volume increases by the cube. Your lungs only work because the tissue is like bunches of grapes because it needs as much surface area as possible to exchange oxygen and carbon dioxide.
Birds are dinosaurs, and as such they share features that help them cheat this a little, like air sacs, and hollow bones.
Also the atmosphere was just thicker because of heavy vulcanism which helps if you're trying to get huge and stay huge.
If you look through the fossil record you see about the max size of land mammals, and they looked like weird giraffe horses.
It's also going to mean you have to move on all fours, being bipedal and the size of a building is going to be rough.
If you ever tried to stand up either your head would explode because your blood pressure needs to be huge to launch blood from your heart however many feet it takes to get to your head, or you would faint because no blood is getting to your head. By the time your head hit the ground it would be close to terminal velocity.
