From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
The difficult aspect of this asana for a fat person is the position of the arms. Ideally, the elbows are shoulder-width apart, but a fat student may have too much bulk in the upper arms and back to accomplish this. Adding to the difficulty is the fact that s/he may have a mild contracture of the deltoid, lateral biceps and lateral triceps. Thus it can be harder both to get leverage to lift the torso and to have the strength to support it (and “it” is heavier, to boot).
To work on this, one can bind the elbows with a strap, then have the student simply do the asana as well as possible. Be aware that s/he must have done some serious shoulder and arm warmups before attempting this asana. Also, s/he will need to be “spotted” to help with the strap.
I always place my hand under the lumbar back to ascertain the amount of curvature, since it’s more difficult to eyeball it than with a thinner student.
Lastly, as in many asanas, holding the legs straight and together may be difficult. A second strap may be used for the legs, but I don’t find it necessary unless the legs spread so wide that the student is unable to adequately use the buttock and thigh muscles to protect the lower back. Still, form is often improved with the strap.