Why insects are so small compared to mammals?
The standard answer to this question is - it's down to the delivery of oxygen to respiring tissues. Insects depend upon a process that is essentially diffusion-driven (via spiracles and the tracheal system) whereas vertebrates have a circulatory system that allows oxygen absorbed in the lungs to be carried via the blood to peripheral tissues. And of course the blood contains specialised carrier cells (red cells) which in turn are packed with haemoglobin which acts as a molecular carrier for oxygen.

The giant dragonflies of the Carboniferous (Meganeura) existed at a time when oxygen levels in the atmosphere were much higher than now (32.5% vs 20.9%), so the diffusion-driven gas exchange system could support a larger body size.