I was surprised to learn during prehistoric times, insects were substantially bigger than they are now. It appears they declined in size as the atmosphere declined in oxygen level.
The change, as you might expect, was gradual mimicing the gradual evolution in birds, which as you might also expect, contributed to their demise through predation.
I wonder if this means plants were bigger millions of years ago as well?
See Science Magazine’s Ancient insect graveyards reveal an explosion in bug diversity 237 million years ago for more information.