Size matters, that's why there are weight classes. I weight 230lbs and I can often smother and bully someone who is significantly better at BJJ than I am if I'm significantly bigger than they are.
Ironman will probably have some good input for you but my recommendations would be to use misdirection as much as possible trying to get your opponent to incorrectly anticipate your movements and inadventently create openings for you.
In order to provide useful feedback it would be better if you clarified specifics. Are you in a full/half guard, mount, side control, etc. Are you able to upa (bridge out)? Shrimp out? If he is bigger then hopefully he is slower and your goal is to create a scramble situation where you can win better positions.
Usually he does full/half mount and sometimes a side but not much. When he gets on top of me he just lays down and I can't move my torso unless he lifts himself. I'm new to this so I don't know much about scramble situations. His slowness comes from fatigue and when he's tired he pretty much becomes dead weight.