If you manage to get unto your side, (or pop out from under that fat bastard) Your opponent will likely get up to his hands and knees giving you the needed space to dive in. When you do, either from the elbow or dog fight keep your body coiled, skrunched making yourself small and stay small the whole time while using your outside arm to go cross body pushing up and away, and rather than under hooking the leg, "C" cup the inside theigh pushing up and away. When you arrive between there legs uncoil and rotate with the leg on your shoulder using your entire body to spread them apart.
Heres the thing....
With big people you are the one that moves, not them. By diving and pushing off of them you can put yourself in prime position
to unleash hell. Staying small helps lots because you spin like a ninja turtle, and ninjas are fast right.
I used to underhook the leg too soon and that would kill the entire sub, they would straighten there leg and I would be stuck underneath them. That is why I know push off the side of there body and "C" cup the theigh pushing up and away.
Timing is crucial, need to fast and explosive with the big strong ones.