I was working with one of my older training partners (not like an old dude, he's in his late 30s) during BJJ class, and the guy is a retired vale tudo fighter from Brazil (he was mostly a standup guy, but he's good on the ground too) and he outweighs me by about 50-70 pounds (depending on where I am in competition preparation).
I realized, while I was working with him in my guard, that there is something really different about working with a guy bigger than you, as opposed to working in a weight classed fight, with an opponent your own size. Since some guys compete in open weight grappling, and stuggle alot with being a smaller guy as a fighter (I know that when I compete, I tend to be the smaller competitor, even if the weight difference isn't nearly that extreme).
The major difference doesn't change between MMA and submission grappling and BJJ, because the principles are the same. It's an issue of joint control.
Against a larger opponent, you have an issue with controlling whole limbs. In my personal experience, I can be going for an isolation move, like an armbar or a kimura, and my leverage won't be perfect and he'll power out of it. This is a really serious problem, partly because of the psychology and partly because you can give up a worse position.
I'm not going to make it sound like you need to do it in a traditional position-submission-sense, it's not that simple. Even a mounted armbar is hard to get on a strong opponent.
The trick, as I've personally found, is to play close to the body. Where as an opponent with your sized limbs will give up a strength advantage as long as you have moderate leverage, a much larger opponent will not. If you are sinking in a kimura on a guy your own size, it's pretty much done, but a larger opponent can punch out of it.
Playing close to the body doesn't mean sticking to chokes, but it means keeping your torso against his, or your side against his chest, in order to maintain a connection and keep your own balance. If you create distance, you are not doing anything for yourself except creating an opportunity to run away. That is not the intention.
Realistically, if you play in to the body you will make it easier to push through his center of gravity, instead of trying to use an outside sweep (like a scizzor sweep), this keeps him from having the opportunity to really use his weight as a prevention tool.
Another good trick I've found, and it's what set me up for the win last night, is to be aggressive. Big guys aren't always used to being attacked, and if you are attacking a submission wholeheartedly and he is resisting, he may make a mistake that gives you the opportunity to better your position or transition to a different technique. It's risky because if he doesn't make a mistake you are expending alot of energy, but, realistically, you are going to do that if you are fighting a bigger opponent.
The tricks that you use as far as set ups for sweeps or alternate submissions don't need to be complicated, just efficient. The one that I have found works really well for me, personally, is the kimura-to-hip-sweep transition. If an opponent, even a stronger one, is focused on getting out of that kimura, they may posture up to try and straighten the arm out forward. As soon as that happens, you are essentially set for the hip-sweep. That also works for a basic straight lapel choke from guard.
Hope that was helpful for all the small guys out there.