The niche is the answer. The world possesses 'gajillions' of micro-niches. Because of their small size, insects use an external skeleton.
Leverage is of the first class type and is more efficient than an endoskeleton.
Superior leverage explains in part the remarkable strength of insects. A linear repetition - a string of beads, rigid within themselves, but movable in relation to those in front and behind. Ever Since Darwin. Morowitz, H. 1982. Beetles, ecologists, and flies.