[El_Phil]

The cap ships main guns may outrange a fighters but such guns wouldn't have the fast tracking you'd need to hit a rapidly dodging fighter. Unless you've got some funky tech. of course. Besides a fighter should be a hell of alot faster than a cap ship, even when dodging and weaving, due to having a tiny fraction of the mass.

Frontline convoy maybe a ship of the line escort, but for anti-pirate duty it would be massive overkill, hence smaller ships. Also for a high speed convoy a DN would/could just slow things down. It depends upon the threat what escort is appropriate, so flexibility is good.

ST tends to keep ships around for decades so once something is built it stays. But you would build the newest design ship in your yards not a xx year old design, hence lots of different class ships that are similar sized.

Small ships for all the reasons above AND why build a BB sized science vessel? AND the Defiant was just guns no exploration role hence alot smaller.