I think I understand where you are coming from. Assuming you are doing it right that is.
I am 180 and fairly strong. There is a guy in our class who is about 125, very skilled guard, but even when he does get me in stuff, even tight, as long as I counter correctly it is actually fairly easy to get out, or if I'm feeling lazy, just stack and really squash him.
How could it be otherwise when I'm able to escape the submission attempts of much bigger, stronger people on a regular basis (with comparable technique of course). Unless your skill is other-worldly you will have problems with big guys.
I agree with the others who have said chaining is your friend. You are small, so you should be swifter than most your opponents, your best chance is to keep chaining and eventually you'll catch something that is so sudden and sunk that there just won't be any time to counter.
On another note, subs that are very leverage effecient are the heel hook, and armbar from the top (where you won't have to deal with their stacking pressure). I have seen little guys cause havok with these.
If you are submission grappling by the way, you might want to think about gi grappling, size isn't nearly as much a factor there-lots more skill (just a thought).