No.
Baseball players are always saying things like “mass equals gas”. I subscribed to this belief my whole life because as I got older I got heavier and I threw harder. The powers of puberty and skill development were conspiring to make me equate mass with gas. All this is to say that throwing a baseball really, really hard is about a lot more than how much you weigh (and even how strong you are).
I think people like to make things simpler than they are (because we are stupid and not good at dealing with nuance and complexity). It is easy to tell someone to just get big and strong and they’ll be better at baseball. This is true up to a certain point. A 5’11”, 130 pound freshman in High School could probably benefit a lot from gaining some muscle mass. However, a 5’11” 200 pound sophomore in college would probably be barking up the wrong tree if he was trying to put on more weight to get his fastball velocity up.
Your ability to throw hard depends on how fast you are moving your hand when you release the ball. Your hand speed depends on the angular velocity of your arm in your shoulder and the angular velocity of your shoulders around your spine. The angular velocity of your upper torso is driven by the angular velocity of your hips and probably some “spring constant” (we’ll get to that in a second) that exists in your abdomen. The impulse from your hips to your torso will be determined by the power you can produce with your front and back legs, and the springiness of your core.
Now along this whole sequence you can “leak” power/velocity if you have poor timing of when all these impulses hit or if you lack the mobility/stability to transfer the energy from one step to the next.
Have you ever seen a mouse trap? I like to think about pitching as a series of mouse traps that go off in sequence. A mouse trap works by twisting a Torsion Spring. If you’ve ever taken a Physics class, then you might remember Hooke’s Law.
Hooke’s Law for a twisty spring says that the torque from the spring is proportional to a “Spring Constant” times the spring’s degrees of displacement from its resting position. The stiffer the spring, the higher the spring constant. The angular form of Hooke’s Law tells us that, if you want to throw hard, then you must have a “stiff” spring that is able to be displaced a lot from it’s resting position. This law applies to all rotational movement and is what drives velocity. Check out Aroldis Chapman’s spring in action:
Check out the same action occuring in the shoulder:
All that being said — I think about mass as being loosely correlated with power. If you are heavy and a pro athlete who moves well, then you probably produce a decent amount of force. This total amount of force you produce is your ceiling and your mechanics and athleticism allow you to access a certain percentage of this total force. I’d also like to highlight, that just because you’re heavy does not mean you are strong. It’s better to produce lots of force than to be heavy — the two are not 100% correlated.
If you’re really powerful, then let’s say you have 100 units of power. However, your mechanics are bad so you only are able to access 35% of your power, so your functional power is only 35 units.
If you have 75 units of power and access 80% of it, then you’re going to throw harder than the person with bad mechanics and lots of power.
Move well. Then add force and stiffness through whatever ranges of motion you’re capable of accessing and you’ll probably throw hard. Be twisty and springy. Also, your body is smart. If you just try throwing something as hard as you can and stop trying to make yourself move a certain way, then your mechanics will probably be better.
Let me know if you disagree please. Warmest regards.