What are constraints?
An evolutionary constraint helps explain why something didn't or doesn't evolve why elephants don't evolve wings, why jellyfish don't evolve a speedy means of locomoting, or why insects don't get to be super-sized.
Land-dwelling vertebrates have gotten big really big, in the case of dinosaurs but even your typical modern vertebrate a lizard, for example is heftier than all but the largest terrestrial arthropods. So why don't terrestrial arthropods get as big as elephants?
Evolutionary constraints on size likely has to do with some of the characters that arthropods inherited from their common ancestor as well as the physical laws of nature.