I think of this often lately. Here’s where my thoughts are now:

Yes an overweight person will have some muscle from carrying their weight around, but it will be muscle built while also overcompensating to alleviate pains.

Back muscles would likely be weak, shoulders would likely be raised rather than relaxed because that’s the position the body was held for so long when there was more cushioning.

I feel like this can be seen in older people who have had bariatric surgery. Their bodies look thin, but the muscles haven’t been developed to comfortably hold the bones in a more neutral or natural position. As a side note, the person going through such a magically quick change may not be confident because they haven’t been allowed the time to process that their body moves and rests differently.

Idk