I'd say one of the best ways I've really submitted larger people is a reverse armbar. Pull an armbar, but instead you flip over. This way the floor helps your leverage and when they try and stand up (If your leg isnt over their other arm like it should be) you can arch your body more and possibly hurt them worse. This is, of course much easier to roll out of so make sure you do it fast and as tight as possible.
Also, never rule out leg submissions, youd be surprised how people let their legs go sometimes.