From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
The only real problem here for some fat people is the need to keep the legs together. I don’t think the student risks injury if s/he has a slightly wider stance, but it does tend to decrease the feeling of the body being in a smooth, compact crescent-moon arc — an important part of the “energy” of the asana.
I don’t have a solution to this, but I do think that if keeping the legs together causes pain or creates added balance problems, then widening the stance is justified.
Anyone, fat or thin, may have a swayback, and it’s important to protect the lumbar area by tucking the pelvis. For a fat person with a very large belly, the lumbar area may be vulnerable even without a clinical lordosis, and in my experience it is safest to teach this asana assuming the need for extra lumbar protection for most fat people.
Also, the additional weight in a fat person’s arms may make it difficult to hold the pose very long.