There is a theory that ideal break cue weight has to do with how your muscles were formed -
how many fast-twitch muscles you have in your body, vs. how many slow-twitch muscles.
See
this link for more details.
Most people are born with about a 50/50 mix. But some have more fast-twitch fibers. I think the idea is that people lucky enough to have extra fast-twitch muscles can generate more explosive force in a short time.
The theory sounds believable, but it's not clear to me which muscle type prefers which cue weight. Maybe people with fast-twitch muscles can move a heavy cue to top speed faster, so for them it's ideal to have 21+ oz. Then again, maybe the slow-twitch people need that extra weight because more mass brings more power to the cue ball, and their arms can't snap forward fast enough to get the same action from less weight.
Arm length may also be a consideration.
The short answer: it's genetics. Try different weights and see.
this actually used to be my thought process on this as well, as I am a rather large human being lol (6 foot 5, 265lbs.)
however, working with Hillbilly for about 20minutes at a tournament in TX a few years back, my mind was drastically changed.
I used to thing that becaues i was "big and slow" that I needed this huge club to smash the balls with... NOT TRUE
and the whole idea of 12 oz break cues and the craziness in that whole realm... meh
what I have found is that ITS AN EVEN TRADE... you can move a lighter cue faster, but it has less mass; you will deliver a heavy cue slower, but the additional mass of the cue will "drive" the ball more easily
the laws of physics are static kinetic energy=mv2 (massxvelocity)
so the weight of the cue only sacrifices ball delivery in one of those two variables....
the key is 2 fold
comfort and efficient transfer of energy
its really that simple...
just like a true well stroked draw shot, its not about how hard you hit it, its about how good you hit it.
i am capable of breaking 30mph+ consistently IF I CHOOSE TO, but its pointless, there is less than no control, and the cons far outweigh the pros.
you are much better off hitting the rack PERFECT at 18mph, than you are crushing the rack with a "less than perfect hit"
I have been fortunate enough to spend a large amount of time on my break and have settled into the 24mph break that squats whitey and usually make 1-3 balls.
thats just my take though... fast twich, slow twich, light, heavy... doesnt matter...
HIT THE BALL WELL!!!!