I'm a little late chiming in on this one, but thought I'd offer my 2 cents anyways. You won't be able to muscle a guy like that around. He doesn't seem like a complete fat tub, so your speed advantage won't help you as much. I'd practice moves that require a weight and momentum commitment and time it right. I'm 5'6" and 200 lbs, so I'm rarely faster than someone, but my timing is excellent. On bigger guys, I usually rely on footsweeps and ankle picks. Lead him around with your sleeve grip by either quick-jerk kuzushi or a lead step away from your sleeve grip. Once he steps forward with the foot on the side of the sleeve grip, hit him with a kouchi gari. If you time it right and have your foot on the way before he steps down, he'll go timber. Make sure to lean towards the rear corner you're throwing towards and anchor down with your sleeve hand and drive through with your lapel grip. The same lead motion can be used for ankle and leg picks. Lead him around. Before his foot hits, change levels (squat lunge) and pick his ankle just as it hits the mat. If he's the kind of tall dude that likes to get a huge over-the-back grip and push you down and wait for uchi-mata, there's a solution for that too. Say you fight right handed (migi). You've got a sleeve grip with your left hand. He throws that hand over your back to bend you down. Now all you have available to grip is from the elbow to the armpit. Grab an inverted fistful of gi like you're doing an uppercut punch, step forward and across his body while lowering your hips, and throw sode-tsurikomigoshi. His fingers will rip right off the back of your gi, and if he doesn't go flying over the top, he'll at least think twice about doing it again. If he gets wise to the sode, then that sets you up for a combo move to the rear with a kosoto-gake, etc... Have your instructor show you techniques for fighting an opponent who's stronger and heavier than you. I'd stick with sweeps like kouchi-gari, kosoto-gari. Ankle picks. Get wise to whatever forward throw he likes and be ready with tane-otoshi or ura-nage.