Originally Posted by Couchwarrior
Just like XitUp said, evolution doesn't always make animals bigger, it just make them more suitable for the current environmental conditions. That's why some prehistoric eras (like the Jurassic and the Pleistocene) has so many large animals, while some eras, e.g. the present, have very few.
evolution is about making animals more well suited to their enviornment. the economy of size is what lead to most animals to be smaller than their ancestors. mammals grew bigger, but humans aren't as big as gorillas. and cold-bloodedness isn't suited too well to being gigantic, so now lizards are small. although, if they didn't have the mass extinction, who knows...