Agreed. The issue isn't what adcoms think of the person's weight. It is whether objectively the person would be able to manage the requirements of med school. Many schools directly ask if a person has health issues that might impact one's ability to do the various tasks required in med school. It is in the person's best interest to be aware of his/her limitations, and seek whatever accomodations might be necessary, and not get into a situation where they are doomed to fail.
Med school is not like college -- you won't be sitting in a classroom after the second year. I would point out that in 3rd year, you will in most rotations be doing walking rounds, which means that you will be on your feet walking through the wards of the hospital for as much as 4 hours in a row, following the residents or attendings, many of whom exclusively use the stairwells and not the elevators, regularly hustling up and down 5-6 flights of stairs. And the attendings/senior residents tend to set the pace, so inevitably you will be hustling to keep up. In surgeries, you will be on your feet holding retractors or limbs for as much as 8 hours in a row; your feet will be sore and your back will ache even if you are in decent shape; probably worse if you are carrying excess baggage. And you will have overnight calls where you will be doing scut all night for 25-30 hours in a row, which can put a lot of stress on your body. So yeah, I'd have some serious concerns for an applicant that was morbidly obese. Not in terms of an adcom accepting them, but if accepted, whether they would be biting off more than they could chew (pardon the bad pun).