The difference I guess is that being overweight has negative connotations. Comments about being dressed up might just be making conversation (and possibly an awkward attempt at a compliment), unless the person takes it as an affront.
Some people (not me) associate Thanksgiving with football. Many families even play a game of it outside. It’s not surprising for me to hear about casual dress codes. For Christmas Eve (church) I do like it if the kids dress up… but truly no big deal if they don’t - their company is a rare and treasured treat.