Have some of you guys even played Rush before? The game mode has nothing to do with player count and everything to do with how the match is structured. Player counts were smaller in BC2 and BF3 because those games were running on hardware from the 2000s. As hardware advanced for BF4, so too did player count. The thing that remained consistent was the sense of progression through the map as the attackers take each base, which provides for a diversity of environments.
Rush used to be played in a sandbox. Now it’s played in an arena. Obsessing over the player count misses the forest for the trees.