Also Iceni is a specialised blockade runner.
Also as has been stated in other comparison threads we have no direct comparison in the size of internal workings, though in ST there are lots of reference points, in FS there are non. for example how big is a capship reactor, a capship subspace drive, how thick is a FS ships hull, how big are the missile stores for the warship launchers, how big are the stores for repairing the ship, how big is the crew,
tl;dr = lots of unknown variables that affect the size of a FS capship
and I could easily go on and without that comparison and/or a reference point in RL technology for most of it there is no way to say in any meaningful manner if the scale of one universe or another is correct.