I am having a lot of trouble with the weight painting. Particularly where the legs meet the torso, I get a lot of deflation and flattening when trying to move the rig into a sitting pose. How do I get the legs to have more volume instead of collapsing in on themselves?
The area where legs meet the torso is notoriously about deflation around the joint. The fact is there's much more happening in that area than a simple bone and weighted mesh. There's underlying muscle deformation and skin "sliding" that enables human/animal joints to rotate without deflation - all that's really possible but really complicated to achieve with computer graphics.
Practically, for you and me, we A) get used to it happening and try to hide it best we can in pose and B) you can enable the Preserve Volume checkbox in the armature modifier on you mesh. That will attempt to fight the volume deflation a little.