Let's not miss the obvious. If technical superiority in the Martial Arts is the equalizer against the big guy, what do you do when the big guy is as good as you are?
Work on your technical game (i.e., marriage of gravity), but strength-train like a mad man too. The bigger you are, the harder you'll hit. And don't forget to train legs...you can't fire a cannon out of a canoe.
My Kenpo sparring partner is a 265 lbs body-builder who is as quick and tricky as any of the smaller, twitchier black belts in the studio (interesting place to be, in front of a fast-moving, clear-minded train). My BJJ randori partner is a state champ power lifter, and it takes him a lot less energy to wrangle me then it does for me to chase him...not to mention he can choke somebody reeeaaaalllly hard.
And learn about the biomechanics of power. Sports physiology and kinesiology journals and texts are full of articles and research pieces on optimum form for generating ballistic kinematic forces (i.e., throwing a ball or shot put). Watch quarterbacks throw for length or speed, or an Olympic javeline thrower chuck one down field...hop, skip, jump, BAM! That's the intensity you want in a rear-hand strike...not just the mere shifting of 60/40 weight distributions in stance, but the whole-hearted intensity of a shot-putter going for gold. More strength; more momentum; more intensity; and get out of the stance box.
Mental exercise: Can you imagine SGM Parker after a couple years in the gym on juice and Growth?