@wilhelm_stenvall I don't think there's any indication that she's struggling with his weight. She seems to take his weight very easily without trouble. She's just flustered that he's sitting on her and she's going to enjoy every single second of it.
@goodhunter Her inner concioussness might also be at play here. It was suppose to be a punishment, so maybe her body is trying to find reasons to be humiliated and such?