No, I wouldn't do that. I would be too embarrassed and worried about hurting their feelings. The only time I would bother my seatmate in a situation like that is if they are leaning on me, drooling on my shoulder or the like. If I can see that they are so overweight that they can't help it, I don't do anything because what could they do about it? Just be glad it happens infrequently. I would rather them be snoring than awake swapping loud stories with neighbors or people standing in the aisle, leaning on my seat etc. ...