From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
As with Balasana, the thigh fat just doesn’t know where to go in this asana! Therefore, the fat student may not be able to sit on the heels at all. For some, however, especially midsized people, the trick I described for Balasana — rolling the muscle and fat on the calves and thighs laterally — may help. Usually, though, sitting on a cushion is required, and the legs will simply be as close together as the fat permits.
Another potential problem in this asana for the fat person concerns the feet. The heels bear more weight with a fat person, and the extreme plantar flexion that results may cause painful cramps. In addition, if the student (fat or thin) has plantar fasciitis (chronic inflammation of the connective tissue in the soles of the feet), this asana can aggravate the condition.
To solve the problem, I use a large enough towel roll under the fronts of the ankles to make the angle of the ankle between 90 and 120 degrees, rather than the more usual 150 degrees or more.