As for the supposedly older ships we see, they aren't noticeably smaller than the Enterprise. They also seem to feature very potent armaments. Several possibilities exist, but it seems probable that Starfleet before ENT was a fighting force, unable to explore anything because the ships lacked the range and speed required to reach unexplored space. The older fighting ships may have featured heavy phase cannon armament for decades already, and NX-01 only lacked it because its installation was delayed by the hasty launch in "Broken Bow". Or then phase cannon were a new invention, and NX-01 was among the first ships to have it installed, but all the older ones had it retrofitted within the year, too (a fairly trivial task, considering our heroes could do it all on their own in the middle of nowhere).
Or seeing as Enterprise got weapons upgrades before heading out to the expanse they just had better weapons becuase they were closer to earth and could get the newest toys installed becuase of it whereas Enterprise had to come all the way back home so they wouldn't get anything developed in the mean time. Remember the parts for the phaser canons were already on the ship and just had to be installed the pulse canons and photonic torpedoes on the other hand had to be picked up at Earth.