[Punisher454]

Yes a Battlestar is much more than just a carrier. I think its an all-in-one supreme fighting ship.
I believe that the primary defense on a Battlestar is mass, lots of it. BSG does not use shields like we see in Trek and to a lesser extent Star Wars. To survive in battle a battlestar has to be big enough to absorb a lot of pounding. This means that there is a lot of usable space inside. The battlestar serves as the carrier, the battleship and the troop transport.
While I do think there would be other combat vessels such as long range scouting ships I dont think colonial battle groups would have large numbers of intermediate sized craft. Most of the ship types we see with earth Naval craft have simply been consolidated.