Why were there so many gargantuan animals in the past, and why did they evolve

[nemo nisi mors]

to such smaller creatures today? In class we've seen pictures of various prehistoric fossils of fishes, reptiles, dinosaurs, and bugs that are humongous. Now, all of their ancestors are so so so tiny in comparison. Why did this change from large to small happen, and why were they so large in the first place?
Whales are pretty big, true (and so are elephants), but those are exceptions which don't mean much in this big world of animals.