From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
I don’t think this asana should be done any differently by fat people. Fat students need to pay special attention to not collapsing the underside of the rib cage because just the bending alone will restrict breathing somewhat.
Also, since the torso of the fat person weighs more, s/he may be tempted to rest a lot of weight on the hand touching the forward leg. This is a no-no for everyone, but especially if s/he has problem knees, it’s easy to make things worse by doing this. Also, if the fat student’s weight is primarily in the tummy, you should watch for swayback.[Note from Gyandev McCord : Some Ananda Yoga Teacher Training students have heard me speak of a very heavy — okay, fat (whew, that was hard to say!) — student whose video workshop asana was Trikonasana. You also saw me demonstrate the way she did it — rather narrow stance, very little sideways bend — and say “But her spine was straight, her pelvis, torso, and shoulders were opening, her breath was full and even, and she was vibrant, joyful. There was no doubt: it was a real Trikonasana!”] [Well, that student was Kay. In the picture above, you can see how nicely her pose has evolved. Can’t you almost feel the energy and joy flooding Kay’s body cells?]