It doesn't; 1GB is the minimum. We have to fit Redis, Postgres, Ruby, Sidekiq, and 2 Unicorns in there. There are a bunch of fairly active instances on 1GB RAM, which works fine. If your community grows a lot you'll need more resources, of course.

It also warns you if you try to use gmail to send mail, because gmail throttles you to only sending 2000 emails per 24 hours.