It's all based on a very fundamental mathematical law: if you increase the size of something, the volume increases with the third power while the surface area increases with the second power. An insect twice as large would be 8x as heavy and need 8x as much oxygen but 4x as much surface area.

That's also the reason why insects are as strong as they are. The strength of a muscle scales primarily with the cross section area of it, which again scales with the second power. So if you'd increase the weight of an ant by a factor 10,000,000 (e.g. 5mg to 50kg), the expected strength would increase by 10,000,000^(2/3) ≈ 46,400. If it could lift 10x it's weight at the original size, it could now only lift about 4.6% of it's weight