Or upgradeable to do the same. For that matter, even if both Galaxy and Intrepid have Type X phasers, capable of outputting X terawatts (or whatever) of power, who's to say the Intrepid has that kind of juice to dump into the phasers anyway? The Galaxy may have more raw power to use for phasers or shields or whatever. Same speakers in the system, more juice in the bigger D batteries, if you'll pardon the pun.
Mark