More hard-points for weapons placements can quite effectively make up for lesser power output, if that was even a consideration to begin with on the smaller ships. A wider variety of firing arcs, when employed by an experienced commander and tactical officer, would likely win out over another, theoretically more powerful ship with blind spots. Since Reliant didn't have experienced people and Enterprise did, it could be argued that they were evenly matched.
Many years ago, when Starfleet Command & SFC II were out there, I played a couple rounds with another gamer, simulating the initial battle between Enterprise and Reliant. I played Enterprise first, he played Reliant and then switched. Both times I won, with either ship, simply because I knew how to fly the console faster than he did (and the Reliant did have greater armaments and a more comprehensive firing arc in those games compared to Enterprise). We even made sure that, in both attempts, the Enterprise approached Reliant with shields down and Reliant raised shields and shot first. Outcome was the same both times. We both agreed that my quicker ability to set up automated ordnance/suicide shuttles to supplement my attack and keep him busy while I made repairs won me both matches. Now, admittedly, that didn't happen in the real battle, but it did prove that experience and speed were the game-changers.
