you could possibly get an armbar, I armbarred this black dude at a party once who was a 3-a state qualifying wrestler who was like 35 pounds bigger than me Lol not that impressive for someone who didn't know how to defend an armbar but shit its not like i have any grappling experience so...it can be done.
if you get their back just act like your going hard for the choke, and when you hook one of their arms, use your other arm to push off their neck so you can swing into the armbar quicker. Cross your feet, with whatever feet is against their neck being the foot that is on bottom, making it harder for them to roll into it and power out of it. if you have a pretty strong core and are quick about it, you shouldn't have to worry about hooking their leg, as it seems like it kinda gives up a little bit of the power, and you would want as much power and to be able to crank on that arm as quick as you could with it being someone much bigger.
Also just work on getting a very powerful squeeze. I'm sure you could find videos on google or youtube explaining how to make it more powerful. According to eddie Bravo, sitting down and bringing your knee close to your chest and RNC'ing it will help it alot, though it kinda hurts your arm if you do it alot in a short period of time.
Either way, its going to be hard as shit to do against someone much bigger. There are weight classes for a reason.