While attacking the Airfield Princess during the final episode, Nagato and Mutsu turn up midway through the fight. After firing a volley of shells at the target, Nagato says out loud, "Don't underestimate the power of the Big Seven!"
What significance does this hold? There are more than seven ships present, so it's unlikely to be that, and the ship Nagato has more than seven guns both in the anime and in real life.
So, what is Nagato referencing when she mentions the 'Big Seven'?