Wukka's video is a good one. My preferred escape against the fat guys, at big guys in general, when it comes to the mount is the upe/hook-and-roll escape. Basically, hook a leg on one side with your leg, control a sleeve on the same side, and then bump your hips.
If he gets to a very high mount so that he is no longer on your hips, and you can't use your elbows to work him down, I often offer the armbar by extending both arms and bridging on to my shoulder. If they take the armbar, forcing the bridge harder will basically always land you on top, and if you've been pinching your elbows together as you offer both arms, he's not going to have a good angle on the elbow to finish the armbar.
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