From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
Half-Spinal Twist is a nightmare for fat people! It’s usually the knee that’s pulled up to the chest that’s the problem. The fat student usually can’t even find out how supple they are in this asana, because the belly gets in the way.
Difficulty breathing, and a tendency to lean back (to unconsciously accommodate the belly) are the usual result. I seldom teach this asana in my class, substituting supine twists. However, I do sometimes use it to teach alignment. (It’s a good one for demonstrating keeping the head aligned with the torso.)
I usually recommend that the fat student raise the knee to a comfortable level only, and that s/he not cross the raised leg over the other knee. This, of course, lessens the stretch in the lower back and buttocks. On the other hand, it permits the fat student to breathe. I know which one I’d go with.