Evolution - small to big?

Doesnt evolution say that modern day animals evolved from smaller animals, just to put it in basic terms at the start there were small things and they became larger more complex things due to evolution Then why is it in the past, animals tended to be...well bigger?Examples
Giant Sharks: Megalodon bout 60 ft. Now sharks are smaller. Giant Crocodile: Deinosuchus about 50 ft long now crocs are smaller.
and giant mosquitoes, giant sloths....[This message has been edited by hoju, 09-02-2003]