Why did prehistoric animals .....

[joe1]

Why did prehistoric animals and insects get so big - and why don't creatures today?

[Clanad]

You've asked one of those questions on which everyone has an opinion, but, truth be told, there are no satisfactory answers.
Here in my home State in the western U.S., dinosaur bones are as frequent as sagebrush and they come in all sizes. But, notably, during my higher education in which I majored in Geology, several theories concerning two subjects oftened intertwined. One was your questions, but another related one played an important part into which school scientists found themselves; that being, were the dinosaurs, especially the large large ones, cold blooded or warm blooded. Simple, but unsatisfying answer is... no one knows for sure re: either question...

[wildwood]

They didn't as a rule, it is just that the big ones get all the headlines. Some were able to be that big because the vegetation was totally different then and there was plenty of food about. No grasslands or deserts, just about everything was lush and green. The predators could also grow that big as there was plenty of prey about. There were many hundreds of dinosaur species smaller than a horse, even down to the size of a chicken, but are not so sensational to the average Joe so are not well known. Small insects such as springtails and minute bugs were around even before the dinosaurs evolved but you never see them in the headlines.

You can just imagine how people in a few centuries (if we are still around at that time) wonder in awe about an ancient creature that grew bigger than any other animal in the history of Earth - the Blue Whale.