Why can't animals with exoskeletons grow as large as those with endoskeletons?

Question

I know that there are limits imposed by the diffusion of oxygen through the tracheal system, and I know that the process of molting would be inconvenient for an animal the size of, say a rhino, but does anyone know any reason, in terms of engineering mechanics, why animals with exoskeleteons cannot grow as large as those with endoskeletons?

Answer 1

Animals with exoskeletons also lack lungs and a true circulatory system, they breathe through pores in the exoskeleton.  so as the size of the creature grows, the ratio of the amount of cells that need oxygen grow faster than the surface area of the animal as a whole.  for example a 1 cm cube has a surface area of 6 cm sq and volume of 1 cm cubed, but a 2 cm cube has a surface area of 24 cm sq and volume of 8 cm cubed, and a 3 cm has a SA of 54 and vol. of 27.   This is true of insects, spiders have early style lungs, but still very inefficient compared to mammalian lung systems.  Exoskeleton animals have gotten to large sizes in prehistoric times, but oxygen contents were much higher allowing larger growth dispute inefficient lungs.

Answer 2

A mobile animal would have to be able to carry the weight of a shell that size, the shell structure would have to be such that it didn't collapse upon itself, and the shell would have to be strong enough to support the weight of internal structures connected to it.