I'm going to give you the instructor's point of view, as you're a black belt and black belts have some instructional responsibility.
In kendo, when instructing a low ranking student, I leave them some openings and pace it so that they can try out techniques and feel like they're accomplishing something. After a little bit, I step up and take a few points just to remind them that that second dan black belt I wear aint just for decoration.
Then I step down to somewhere in between, which generally brings out their best; they know that they can execute the technique and when I step it down a half step, they have an attitude that they need to defend.
I praise each correct block, strike, or exceptionally well done technique. I correct them as we spar, usually addressing their biggest failing and one or two minor failings. Thats usually all that a beginner can handle.
With intermediate students, I tend to maintain the intermediate pace (my step down pace) the whole match and let the pieces fall where they may. Older teens and anyone above blue belt I really don't coddle a whole lot. They're as big as most of the adults and have the vigor of youth. At blue and up, they've been around long enough to understand what it means to step into the ring with a second dan blackbelt. Generally, they're disappointed if they think I've gone easy on them.
So to bring that back to you, Manny, make it a positive for both of you. You have the maturity and the strength to dominate the match and you're a strong opponent. For large teens who hit hard, remind them of that fact. And when its all over, thank them and tell them that they did well. Generally, the big teens are competative and on some level, relish the thought that they can hold their own with the big guy.
Lastly, if they whine, remind them that they're learning to fight, not to dance. You get bumped, banged and bruised in a fighting class. Occasionally in Taekwondo, the large teens will deliver hard kicks, miss the hogu, and then ask if I'm alright. I always tell them that I volunteer to let another fighter punch at and kick at, and in kendo, swing a stick at me. Bruises and bumps are a logical consequence thereof.
Daniel