Looks like BJJ would be a good suit for you!
This popular martial art deriving from Japan via Brazil will help you complete your skill in throwing, grappling and submissions, both standing and on the ground. The techniques you learn will lead to an improved understanding of taking an opponent to the ground with a series of positioning, pins, escapes, sweeps and submissions via joint locks and chokes applied in order to gain physical advantage in ground fighting situations.
The premise is that most of the advantage of a larger, stronger opponent comes from superior reach and more powerful strikes, both of which are somewhat negated when grappling on the ground.