Its not your imagination. We open worlds when the previous one gets very close to full. Omicron has a cap of 20,000 players and Grepostats shows we are at 19,250 players.
It wouldn't be very nice of us to turn away new players to the game and once a world gets more than a month or two old, it is not reasonable to have a new player be able to compete. It means more work for our mod team, but we will happily release worlds as they are needed. Now to expand the mod team some more.
Yeah, I guess there is no real perfect solution, but the nice thing is that if you don't like these settings, there will be a new world in a few weeks.