Mark Rubin has also explained as to why current gen platforms do not support 18 player matches. He explained that due to the resources taken up by the new engine, current gen wouldn’t work well with 18 players.

Mark Rubin: Honestly, we played with it for a while, and just felt, performance wise, it wasn’t a good experience on current-gen. We had done enough new stuff to the engine, added a lot of new features, that the current-gen systems became bogged down under 18 players.

Yeah, it sucks. I know a lot of people play Ground War. But it’s funny how many who play Ground War always complain Ground War has issues. I’m like, well, it’s because you’re 18 players on a tiny little map. Of course you’re going to have spawning issues.

That being said, people still enjoy it. And we totally wanted to do Ground War on current-gen. It wasn’t an easy decision to cut it. But we wanted to make the game good. Character customisation, for instance, takes up a massive amount of resources. So having 18 people on the map with unique characters is a challenge compared to having 12 people on the map with unique characters.

SOURCE: EuroGamer