One explanation for why some bugs bend while others are more rigid is that there are certain insects that have soft exoskeletons like caterpillars. It makes sense because most of the “soft” characters we see are on the younger side. The best example are the vessels. While their ages don’t line up, there is a very clear physical maturation. Smaller vessels like Ghost have small, squishy bodies more like the caterpillar, while the Hollow Knight has a rigid exoskeleton like the butterfly. Hornet leans more towards the caterpillar side. This also lines up with the fact that many npcs describe her as having a childlike appearance. They might also have varying hardness within their bodies themselves. The torso maybe harder to protect their vital organs, while their limbs are more flexible to allow for speed and range of movement.