We saw one ship. Earlier Khan says he was developing "ships (plural) and weapons." But of course you're just deflecting because a smaller and less powerful Starfleet ship would not have fitted the story of ID at all. And as for Wrath of Khan's Reliant, lots of blueprints and manuals available after the movie's release (which called it an "Avenger-class Heavy Frigate") talked about "megaphaser cannons" which were more powerful than the Enterprise's phaser banks, although I don't know if that was based on behind-the-scenes movie notes or was just from the blueprinter's imagination. Reliant definitely had aft-firing torpedoes, which the Enterprise seemingly didn't.