A ship's firepower in this system is an abstract represenation.
For example, Defiant probably has the highest foreward-arc firepower in Starfleet... but almost nothing to the sides and rear. Her firepower in the game takes into account the fact that she often will not have a target upon which any of her weapons bear.
Excelsior's weapons are individually not as powerful, but she can fire her phasers more-or-less equally well at a target anywere in range. Larger and/ or military vessels also likely carry greater amounts of expendable ordinance, and presumably are more generous in it's expendature... AFAIKR I've only seen Defiant fire two Q-torps at one time... Enterprise-D has fired spreads of up to five P-torps, and we know that Voyager started with only 28 P-torps (IIRC).
I'm actually thinking of using CODA Starship combat with GURPS:PD rules for the RP part...
“I am a soldier. I fight where I am told, and I win where I fight.”
General George S. Patton, Jr.