Why do many mites, when pumped, increase in size?

Ticks have an expanding belly because they have special muscles for this. Therefore, from a large amount of food consumption, their abdomen increases. And also females can drink more blood than males. This is due to the fact that females have extensible chitin, which means they increase in size. A female can suck out an amount of blood that is 100-200 times greater than her body weight, while its volume increases approximately 120 times.