From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
This asana is usually not harder for fat people than for anyone else. However, there are a couple of items to keep in mind.
One, usually the hardest part of the asana is lying prone with the head on the ground. It’s hard for a fat person to breathe that way, since some abdominal and chest fat is compressed upward. Once one enters the asana, it stops being a problem. Two, it is very difficult for a fat person to keep the legs together.
The normal difficulty with this is exacerbated by the need to tighten the buttocks and press the pubis and thighs into the floor. This “squishes” the thighs, and once again, the fat has to go somewhere, and it ends up on the sides.
I haven’t found that spreading the legs slightly creates any problem as long as the buttocks are tight and the pubis is pressed to the floor to protect the lower back. However, one may use a strap to help keep the knees from separating too far, if necessary.