Various reasons.

Speaking specifically of insects -- the largest insects lived during the Carboniferous and early Permian. This is due to two reason. One is that oxygen content in the air was higher back then. Insects do not really have an advanced respiratory system. The second reason why insects grew to such large sizes during that time is because they lacked predators. They were the predators. Studying the history of insect sizes, we can see that the evolution of birds during the late Jurassic disrupts the relation between insect sizes and oxygen content. Even during times of rising oxygen, insect sizes remain smaller because when such a specialised group of insect hunters exists, being big isn't an advantage.

Now onto other creatures. Humans right now are bigger than we've ever been in our evolutionary history. We evolved from really tiny proto-primates, the size of squirrels. If we are looking just at our evolutionary history, we've grown tall. That said, there are limiting factors to being tall too in terms of physical health and resources needed. Humans are not very likely to grow much taller, because there is no real evolutionary pressure for us to do so. Often times in history when creatures have grown large, it was either to become a more efficient predator, or to become too big to be preyed on. But neither of those things really apply to us right now. We've shaped an environment in which our size doesn't really contribute much to our evolutionary success.

Now, as for other animals -- The age of dinosaurs really speaks to the imagination in terms of the size of creatures, but actually the largest creature to ever exist lives right now. In the ocean. The Blue Whale is the biggest thing that has ever existed on Earth. As for land animal sizes, Dinosaurs had some advantages that allowed them to grow that big -- hollow bones and airsacks means they were able to reduce their weight even when growing to enormous sizes. They still weighed massive amounts, but nothing like what any other animal would've weighted if you had sized them up to that size.

Right now, we do not have much surviving mega-fauna. And to be blunt, the reason for that is us. We are destroying the natural environment at a huge rate. While the change in climate at the end of the ice age had an effect on animal sizes (or more accurately, the extinction of several huge, cold-weather adapted animals), the way we are living now pretty much ensures that nothing as big as the animals that have once lived (except, again, the Blue Whale in the ocean, and we are threatening them too) is going to evolve again as long as we keep acting as we do. Being huge takes a lot of natural resources, and we aren't leaving any for anything but ourselves.