From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
The difficulty for fat people with this asana is the tendency for the legs to splay open when bringing the feet to the buttocks — another case of “the fat has to go somewhere.”
Holding a strap around the feet, and fastening another one around the lower thighs (don’t suggest this last unless you’re sure the strap will be long enough for the person) can help with this problem.
However, I still allow a few extra inches even with the straps. It’s critical that the student be able to protect the lower back by tightening the buttocks and pressing the pubis to the floor, since the wider apart the legs are, the less protection there is for the lower back.