Thinking along the lines of the Black/Green Dragon conflict: if, as suggested above, the evil wizards have driven off one of the dragons because its lair was too close to their base, it would be logical that it's now vying with the other dragon for territory.
I like the idea of the wizard's base moving: a floating island! (Not to mention locating it in the middle of an ever-shifting series of waterways, caused by clumps of reeds/mud/whatever floating around.)
Another consideration is that in a huge area that has one type of environment, the animals/monsters will tend to grow bigger and bigger (as a previous poster mentioned, more HD).