I think that fighters can fit into a spectrum instead of just being categorized as straight light/medium/heavy. So, from lighteat to heaviest, we have:
Ultralight: Hornet, Ferret
Characterized by very thin shields and armor, and a minimal missile load.
Light: Arrow
Characterized by emphasis on shields over armor, and a missile load that allows engagement of heavy targets.
Light/Medium: Banshee, Pirhana
Characterized by maximized armor without sacrificing high speed.
Medium: Hellcat, Tigershark
Attempts to balance speed and attack power to produce a multipurpose fighter. Unfortunately one often ends up with too much of one and not enough of the other.
Medium/Heavy: Rapier, Excalibur
Packs more attack power than Medium fighters, but not necessarily more shielding and armor. Emphasis in on speed and attack--quick in, quick kill, quick out.
Slow Heavy: Thunderbolt, Vindicator, Shrike
Emphasis is on attack and defense power, not speed.
Fast Heavy: Morningstar, Dragon, Vampire
All the stops are pulled out here in an attempt to maximize speed, attack, and defense. Not always the best in any one category, but always better in all three than medium fighters.
Heavy Bomber: Broadsword, Longbow, Avenger, Devastator
Designed specifically for taking on heavy targets, the heavy bomber has lots of missiles and torpedoes, medium-to-heavy guns, and super heavy armor. Speed is sacrificed in favor of additional protection under the philosophy that it will be faster to simply kill your enemy than to try to dodge it when you have such a heavy weapon loadout.