From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
A fat student will not be able to bend forward as far as a comparatively flexible thinner person, again due to the size of the belly. Bending the knees aggravates this problem, so be on the lookout for locked knees in your fat students.
Another related problem is difficulty breathing. Taking a deep, belly breath may move a fat student’s torso up a few inches, and then it will move down again with the exhale. This produces a slow undulation that I think is not harmful as long as it is not a bounce that would stress spinal discs or knees. Besides, it’s proof that your fat student is breathing correctly!
The fat person will be tempted to open the legs to accommodate the belly. This provides a legitimate stretch in itself, but not the same as Padahastasana. I therefore encourage my fat students to keep the legs at the width with which they perform Tadasana, and to think of lengthening the spine rather than bending over as far as possible.
However, I often have them perform the asana with legs wide apart afterward, to demonstrate both how the belly gets in the way in the proper asana and how well they can usually do the asana without belly problems. Obviously, this strategy could backfire if the student has difficulty with forward bends in general. Otherwise, it’s a fine morale booster.