But during the dinosaur era even insects were huge compared to today.
No. They were similar in size to today. They were far larger before the evolution of the dinosaurs.
Is there any reason to believe that 'microscopic' has always been the same thing in terms of size?
Is there any reason to think it hasn't? We're talking about animals here, also. Average size fluctuates over extinction boundaries.
Doesn't a human increase in biomass as it grows, reaching a peak and then falling into decline with age?
That is ontogeny, not descent with modification.