From the Kay Erdwinn’s Post Teaching Yoga for Round Bodies
Most likely the fat person will prefer Ardha Siddhasana, in which one foot is placed in front of the other rather than on top of it. As with any cross-legged position, the fat person may not be able to fold the legs as snugly (i.e., get as much knee flexion) as a thinner person.
Some very large people may not be able to cross the legs at all. I have these students place their legs in as near to a cross as possible, and support the knees with cushions. This posture makes it tempting to round the back, so I make sure they are sitting well forward on a blanket or cushion so that the pelvis is tilted forward.