Re: Larger/stronger opponent
Besides all the stuff like working on proper breathing, posture, etc...two things come to mind:
1. if he is outright resisting you to a degree that you can never move, then you aren't able to actually learn anything. That's different from simply being harder to work with.
2. for gripping large hands or wrists in nikkyo or sankyo: I have very small hands and weak, arthritic thumbs. What I have been counseled to do is grab uke's fingers instead of the whole hand. It can be very effective.