Bottoms have different ways to get an orgasm: With stimulation on the penis, without stimulation on the penis, with the stimulation of the prostate (the anal orgasm) and/without cum.. here some comments about it. Do you have a comment?
When I was young, in my twenties and thirties, I would have an orgasm without touching my penis. In my forties, it happens less, but it still does happens. I agree though it is as much a mental thing as a physical one. Also size can play a part, but there are other factors as well.
Take care