Is the Defiant a pocket battleship or a destroyer?

Does the Defiant's concentrated firepower make it a pocket battleship?

No. It remains a destroyer, albeit one built around cruiser-scale weapons and propulsion.

The Defiant remains a destroyer despite its very heavy armament. It carries a Galaxy-class vessel's weapons and propulsion with only enough additional structure to support its crew. That concentration makes it formidable, while its small platform and tactical role still resemble a destroyer more than a capital ship. Weapon power alone does not make a small combatant a pocket battleship.