From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
Entering this asana may require some extra “arranging” for the fat person. The arms and legs may need to be wider than with a thinner person. Mostly, though, fat is somewhat mobile (the jiggle effect that we so love in Jello, but for some reason hate in people… go figger), and the fat student may have to squirm a bit until it is distributed evenly underneath and laterally.
The feeling is similar to sitting on a cushion, but missing slightly, so that you are off-kilter. You need to rearrange the cushion to be seated squarely on it. With some experience, I’ve found that I know how to sit, lie down, or whatever in a way that minimizes the need for adjusting, but for a fat person just getting used to tuning into his/her body, there might be a fair amount of restless movement to get things “just right.”
In addition, a person with very large buttocks may need a cushion under the knees even in the absence of lower back problems, simply because the extra fat adds curvature to the lumbar spine. Some fat students may prefer to rest the legs on a chair seat.
Finally, because a fat student’s neck will tend toward the jackknife position (due to the fat pad on the back), a cushion under the head may be in order. Also, a neck roll may prove valuable in keeping the neck in a comfortable, supported, neutral position.