I remember reading something years ago that has always stuck with me regarding the FS class names. FS cruisers are roughly the size of modern naval cruisers - a bit bigger, but the same order of magnitude. It's not outrageous to assume that the initital naming conventions for space warships followed the modern convention - small frigates and destroyers supporting large, powerful cruisers.
Then, human technology advances and our space vessels become more sophisticated. Fighters start to become more powerful, and carriers need to scale up to fit more fighters. These carriers also need to defend themselves, but they're still dominantly carriers despite maybe also mounting heavy weapons.
Then, someone decides to go all "Britain in 1906" and build a massive ship capable of carrying enough fighters and weapons to easily eliminate cruisers. A "Cruiser Destroyer", if you will. We know from the Colossus getting the green light that "Do the same thing, but much much bigger" is considered a solid strategic construction decision in the Freespace universe, so it's not an unlikely course of action, and would be a fairly logical progression to the apparently illogical end state we have now.