I read that before the flood there was a huge water vapor canopy over the earth, which regulated temperature and prevented much more UV radiation from reaching the earth. As a corollary, much more of the earth was land at that time -- people could easily move from Europe to North America, for example, using only land routes.
Less UV radiation meant less aging, which is one of the reasons why men used to be able to live 900 years *before the flood*.
There might have been similar issues with gigantic beasts being able to survive pre-flood but not post-flood. It might be because of atmospheric pressure, etc.
The largest creatures alive today are whales -- things under the sea. On land, they are crushed by their own weight. If there had been more atmospheric pressure pre-flood, I believe larger animals would have been possible.
Matthew