Out of HS, son was throwing over 90 at about 170lbs.
Take a look at Florida's Brady Singer and Alex Faedo. Their velocity doesn't come from their "arms". At full maturity and another 4, 5 years, they may or may not be throwing harder, but basically will have filled out and their bottom half, repeatable mechanics, strong core, and flexible upper half will probably determine added strength.
Have you ever seen a muscular successful ML pitcher IN THEIR UPPER HALF (edit).
I do believe this is what sultan was referring to and Rob T did a good job job explaining.