Marshall Smith of BroJon Gazette has an interesting argument for giantism in previous flora and fauna. It's that the earth used to spin at a faster rate and that various large meteors have struck at such angles as to slow it down. It's a rather complex argument, one of his more logical ones.

It also involved the earth roating on a flatter plane to the sun and that most land masses were around the equator, because of this faster spin and that's the reason cold blooded animals and evergreens flourished, because there were little seasonal effects. So that when the earth was thrown off this axis, warm blooded animals and deceduous(sp) plants had the advantage.
There is some serious evidence for dinosaurs having lighter weight bone structure then would be necessary for their size in today's gravity.