Consider this:
Insects, crocodilians, turtles, sharks and snakes all predated or coexisted with dinosaurs. None of these species' biophysiology has changed significantly since that time and they are bound under the same laws of physics and limitations of available resources as their ancestors, yet all have examples of enormous specimens in the fossil record. After the asteroid hit and the food chain collapsed, the extinction of the larger forms was a given, but we have had 65 million years of recovery, yet none of these species has ever returned to that size or proportion. The largest predator on the planet currently is a mammal, the sperm whale, and look what it has to go through to get its prefered prey. It is only able to do this through extreme adaptation and the ability to exist from its fat stores in lean times, something that many other species can't do. If it were easy there would be more species that were that large on land and in the sea. None of the other largest animals on the planet currently are predators, even the largest shark. The largest animals 100 million years ago weren't predators either. The largest land mammal we know of that ever lived was at least twice the size of an elephant (based on available evidence) and where are examples of that size around now? Oxygen plays the greatest role in the ability for animals to grow large and proliferate, not the other factors that have been stated. None of those other factors has changed as they are universal laws that affect all life on all levels and in any era.