If you want the non-canon answer (and I'm sure many of you don't, so look away if tie-in comics make you puke), the Narada was so badass cause before going back in time but after Romulus blew up, it was upgraded by a Romulan military station with experimental tech.
Which was based on salvaged Borg technology, which eventually fully asserted control over the ship and went off to the Delta Quadrant on a spiritual quest to find V'Ger.
I prefer to leave those comics out of this, but I do agree with the idea that Nero obviously upgraded Narada from its mining ship origins. After all, he didn't intend to go back in time, that was an accident. If he intended to attack the 24th century Federation, he'd have to get past 24th century Starfleet and that would require something more impressive than a "simple mining vessel."