Bone Angles >90° Shrink my Mesh

Hi!

In the models I've rigged, I've got the problem that when the bones bend too far, the vertices get spaced out so much that the mesh is losing volume:



Two questions:

1. Is there a name for this effect so I can google it?
2. How do you avoid it? The mesh already has a fairly high resolution, so I'm not sure whether I my weight painting just plain sucks, if I should add additional bones around the joint or if there's another solution.

 

My impression is that you can heavily improve it by redoing the weight painting. It seems that the weight painting is too heavy for parts that are outside of the range of the bones.

 

Agreed, there's definitely room for improvement in my weight painting.

But if I, for example, only assign the vertices around the model's buttocks to the leg bone, there are even less vertices stretched around the bend and the character is essentially butt-less when sitting

What would help me immensely would be to learn about other artist's solutions to this problem. Thus my hope that it has a name that can be googled.