If you fit a battlecruiser style ship with the Galactica weapons it would lose because the galactica has more of them AND a fighter wing to support it. Why build 300 cheap warships when you can build 120 that can devestate those 300?
Basically this is not Star Trek where the ships zoom about like mad if you look at the New Galactica Universe ships seem to fight like ships of old where they stand and deliver their firepower in volley after volley. A BC wouldn't stand up to the Galactica or any other Battlestar for that matter. When you are facing a ship bristling with flak guns, missile tubes, and heavy rail cannons that fire HE rounds and that ALSO carries a fighter wing then your pretty well boned unless you outnumber it by a good ammount.
Now Unrep and Support vessels are NOT warships they are support vessels and I'm sure the Colonial fleet had a few of those.
As for size vs firepower let's use SEIV as an Example usually my cruisers have few guns then the enemy ships but I usually end up winning because my cruisers have more Shield and armor systems. So why would I build a dozen escorts for the same price when my cruiser could blow through them like snot through a tissue?
Now I have also completely destroyed a light cruiser with a simple destroyer, not because I stood and fought but BECAUSE I outfit all destroyer and smaller ships with missile tubes only so the enemy can't get my range and destroy them, only when I get up to Light Cruiser do I make gunships with shields and armor. But the second I get a full gunship Light Cruiser against a Full Gun cruiser the LC dies hard but still dies, if it gets lucky it cripples the enemy enough for the next poor SOB to win. Now these cruisers or even light Cruisers with shields and armor combo's just shrug off missile attacks and still manage to close the range and destroy your missile ship so size does matter
.
Now if it was a Dreadnought that you had equipped with a few missile tubes and THEN with lots of guns shields and armor it would all depend on how many PDCs your enemy had and how good his shields and armor were. But a dreadnought even equipped with all guns could wipe the floor with a BC without much trouble.
Now Galactica has everything you could want, Missile Tubes (as mentioned by the XO and CMDR alot) for long range pummeling, she has heavy Rail cannons for medium range fire (as we saw them establish a perimiter against the Raiders) and she has a crapload of deck guns that fire flak and Phalanx style rounds for close in. She pretty much IS her own battlegroup and if enemy fighters slip in she just launches her own wing and blows em up
.