Thank you for commenting @doopfenel! You are right, back then there had been very big insects but to adapt in their changing environment, through time they become smaller. What are the reasons for these adaptations can be read above. Some millions of years ago the oxygen was very abundant that is one of the reasons why insects were really big. But as time passed the oxygen becomes thinner and they need to evolve into smaller species because their respiratory system is terrible. Their spiracles are very thin so it cannot expand like the lungs.