As someone who took such classes as an art major, I can assure you they are not recruiting their models from the Playboy agency.
I took art classes at the University of Washington 40 years or so ago. We had a nude model sometimes. No one was lewd about it but a few made quiet comments about her being chubby. But in the college years back then everyone was very critical of themselves and others.
Actually, back then the university being half funded by taxpayers would not admit anyone with very serious weight problems believing it was not a good investment health wise. So students were generally pretty fit and could get snide about those who were not
Of course, now all kinds of diverse bodies and intellects are admitted.