Not just portrayed, though. Defiant herself was able to take on a Keldon class cruiser single handedly later in that very episode. With a complement of quantum torpedoes and those pulse phaser cannons, actually the Defiant routinely demonstrates itself to be a "tough little ship" with on hell of a right hook.
What do you think for the modern equivalent? I always compare it to those Pegasus class hydrofoils the U.S. Navy used to have; now just add a twin-barrel eight inch gun to that hydrofoil and add forty reloads for its missile launchers and you've got tiny warship that moves really fast and carries as much firepower as a heavy cruiser.