I never use memory alpha, or beta, I find them poor resources.
But to use more clean examples. Think of how within less than ten years, you compare the computer revolution of early 1990's to the late 1990's, the same could have happened between the galaxy and intrepid....it's viable that the intrepids computer core is a slimmed down upgrade fo the galaxy. Virtually everything on the intrepid computer wise seemed to agree to that fact.
On another note, mass has little meaning in space, so a smaller warp core and nacelles dealing more speed? That suggests, another evolution in advancing technology.
Still overall, in battle Galaxy is more a hard hitter than an Intrepid. An Intrepid is an advanced scout/explorer and science vessel.