FreeSpace cruisers are bigger than biggest modern aircraft carriers in both
length and width but especiall volume...
EDIT: Well, not perhaps in length when it comes to Fenris/Leviathan and Cain/Lilith. Rakshasa is kinda thin too, but this shot is a pretty good one. The thing is, I can't imagine the cruisers floating so they are at the very least a lot heavier than aircraft carriers...
At any rate I would say the reason for cruisers being smaller than destroyers, corvettes and frigate (Iceni) is the same reason that fighter/bomber wings are smaller than squadrons, while in our designations "Alpha Wing" would be Alpha Flight, being part of a Squadron, which is part of a Wing. It's either
didn't know the accurate military terminology of unit sizes (and ship types) in relation to each other, or (more likely) just ignored the modern definitions and went with what sounded cool and worked in the script and voice acting.