I don't qualify as an expert, but just some emperical information from my son's experience:
His Sophmore year, he was 6'3" & 185 lbs, with his FB at 82-84. During his Soph and Junior year off-season, he hit a rigorous training program, including high protein nutrition. Weights, balance platform work, yoga, core strength training ... alternating workouts 6 days per week so his body had a recovery/rebuild day for each type of workout. Yes, he got his arm much stronger, but he really put on a lot of muscle mass into his legs, chest, rib cage, back, and shoulders. And with the yoga, maintained balance, flexibility, and fine motor control.
By his Junior year, he was 6'4"+ and 210 lbs. Not only could he build up more momentum through his body and carry it down the hill ... he was strong enough throughout his core to DECELERATE that additional momentum. If you want to drive a fast car, you need BIG brakes! Same with deliverying arm speed. Your body has to be strong enough to decelerate that momentum without tearing up tissue in your arm, shoulder, falling over, etc.
His FB velocity improved to 87-89. His goal for next year is to continue his training regime and get up to the 220+ lb range, continue to refine his mechanics and delivery so all that additional strength is working in unison. Hopefully push his FB consistently over 90 next year.
The other thing: all that core strength also helped his power at the plate. A hit where he 'missed' the sweat spot last year as a soph was a caught fly ... this year was a hit to the fence. If he "pi**ed on it" this year, it was flat gone.
Most of the time, your autonomous nervous system won't let you build up more momentum than you are capable of decelerating ... otherwise you'd fall over flat on your face all the time. Some people can consciously push their body to it's limits on every pitch to get that extra couple MPH ... but that's when you start to see injuries. Your body needs to be strong enough to comfortably absorb the deceleration of that extra MPH without constant overstressing to max capacity.