When I used to play American-style football, I noticed that the faster I was going when I hit someone (I played defensive line, but this was mostly during drills) regardless of size, the less it seemed to hurt me.
Why?
Maybe beacuse you were going full out, you (subconciously even) got into a better tackling position and hit more with your shoulder pads instead of your head?
Getting up speed tends to help one lower their shoulders and square away. The interia is helpful in getting the head lowered and taking the impact with shoulders square.
The shoulders are easily the most heavily padded area in American football. ( I tried to post this before jk, so we both posted it w/out knowledge of the others post.)
Running slower, it’s harder to get squared away and the torso/sternum tands to take more of the force…and that is a poorly padded area.