It's a tough position to be in and being a bigger guy myself, it still sucks to be under someone else my size or bigger. Leverage is one thing to remember when in that situation and while it's easy to talk about it versus doing, it's just something ya gotta drill and prepare for. Try to get your hand in his armpit and maintain control of one of his arms, preferably the opposite of his strong hand (if he's left handed, go for the right side and vice versa). Shift your hips like you're going to pull out and roll to that side while pushing in on his pit area. It's a one shot deal but it works more often than you'd think. Good luck!