While many would agree that the Colossus was the embodiment of bad naval mentality (Bigger is Better went out of style a loooong time ago), I personally consider the Colossus to be the 'Little Behemoth that Could'...
...You have to remember that, as some have said before, the Colossus did exactly as it was meant to do, it annihilated the current threat of the NTF, and held its own against a large scale (yet somewhat preliminary) Shivan Incursion. You cannot escape the fact that the Colossus proved admirably in what it was designed to do, save the GTVA (albiet for only a short time). Sure, a fleet of 30 Hecates would have been the better choice superficially, providing better mobility, customisation, and saturation of the enemy, but really, how would they have faired against 80 Sathani, no better than the colossus, and instead 30,000 of the Colossus crew dead, you'd have a whopping 300,000 Deaths! I don't know about you, but that's a pretty big number of Deaths...
...Moreover, you have to consider that the Colossus was built, not only a super-weapon against opposition to the GTVA, but as a major Morale booster, which it succeeded in doing quite admirably. I mean, who here didn't feel a little bit more upbeat and just plain awed when you first played through FreeSpace 2 and saw the Colossus blow the stuffing out of the NTF. 30 Hecates may have lifted the spirits of the GTVA, but not nearly as much as the Awesome scale of the Big C...
...Basically what i'm saying is; if you're examining the effectiveness of any ship, especially the Big C, you have to not only look at the Military aspect of its role, but also the Political and Social aspect too...