Lot of diffident good advice above. I rolled with mostly way bigger guys, I'm 155. If you really want to break him down and use rubber guard good advice above. I personally have better luck working from what are traditionally thought of as more dominant positions so I'd be trying to escape or get to butterfly and sweep but thats just me and I'm a newbie.
No matter how you look at it, you are pretty much going to capitalize on his arm being on your center line and make him pay. You'll have to stay calm though the discomfort and not try to muscle your way to the next micro step. With bigger guys crushing you in any way it seems like you just have to rock and wiggle until you get them to slip up. Just dont try to stay there and weather it cause it will wear you down, but you cant power your way through ether so you have to capitalize on their mistakes and they wont make any without movement, so even if its just to keep things moving try rocking and moving your hips. If you are twisting your body toward the same side you are trying to push his arm toward, it will help it slip off your chest, dont be flat.
Zombie or any swim type move to pretty goddamn hard to do when a guy has 20-40 lbs of muscle on you, got to be fast and then capitalized on before they just battle back.
In my stoned imagination it goes like this; Im being smashed in the solar plexus by some gorilla, it s really uncomfortable, actually it hurts a lot but i dont entertain that thought.
His right elbow on my solar plexus, his right hand on his forehead.
I wiggle and work trying to get to my left side while grasping his right wrist with my left hand (softly), as I get to my left side a little more he's really digging into my solar plexus to stop his elbow wanting to slip off. And now I don't care about applying something a little hard to make it work. Make it a two on one hand fight by palm striking his right wrist with your right hand while pulling with your left hand, trying to get to your left side as you drive his right arm back, setting up a figure four. Trick is switching to both hands, pushing the right arm back and then plunging your right arm behind his right tricep fast as hell! It might lead to a sweep or at lest will let you get to your side and out from under the elbow. I count my victories against big guys in tiny increments like that.
that's right I said palm strike.