I practiced some ground work with this one guy who was extremely overweight and nothing I knew how to do could get him off of me, I was wondering if there are any techniques you could actually use to lift off someone who is about 280lbs other than just pure strength.
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.