What factors might limit the maximum body size to which land vertebrates can grow?

Maybe people held some of those land animals in captivity so they don't grow as much as they would in the wild and maybe people had hunted those land animals so the numbers of males was deducted so that it would be harder for the females to have babies and if they did, maybe they would be smaller because they didn't get enough nutrients because people hunted other land animals that those animals needed to live or survive.