Back to Ward for a second, would having a slight build cause you to maybe tackle with arms, more than "drive through" a guy, so you absorb less of a collision as a survival instinct kind of thing? Again, I don;t know just asking for info.
Honestly, I think a lot of the time it's secondary collisions/hitting the ground/friendly fire causing concussions more than initial impact on a tackle. There are lots of variables on every impact happening on multiple planes so hard to say either way is "better" for concussion prevention. Are you more likely to catch a knee in the earhole while arm tackling? Maybe. Maybe not.
It's more an issue of changing direction than force of impact in one direction, and how that force is dissipated. Hitting someone hard and their going back dissipates the force more than hitting the ground with less force but the ground not giving. It's kind of about rolling with the punches. Don't want to get caught moving in one direction then forced to go the opposite (or in the case of the ground an instantaneous stop.) That's when your brain bounces off the inside of your skull.
The dissipation factor is part of the reason players are taught to keep their heads out of the way when tackling. The body is designed for impacts elsewhere to have more give and not be fully expressed on the head, there are "shock absorbers" in the neck to blunt impacts as well. Unfortunately, when the head itself hits an "immovable object" (i.e, the ground) squarely at speed, bad things happen.
When a receiver ducks his helmet as you try to tackle him and you have to pull off as a DB it leaves you in a precarious position. The WR has the momentum. The DB is the one that is likely to be changing direction at the collision. And likely to be the one hitting the ground harder and "cushioning" the WR.
In some ways I think the seeming "protectiveness" of helmets causes concussions. Offensive players ducking into contact is problematic. The NFL has started to throw flags on RBs "lowering the helmet" a bit more recently. Sometimes I do wonder if the old school leather helmets were actually safer. Not so much in absorbing hits, but in leading players to avoid taking them to the head.
