It may also come down to how many hours of work can be used to make each pack. One team designs the gameplay, another creates objects and CAS items, another builds worlds. There's somebody counting up all the hours needed to do those things and decides what's too much.
When you think about ROM, Glimmerbrook may have been small because they had to spend a lot of time creating The Magic Realm.
But why Willow Springs has 3 empty lots and Evergreen Harbor has... none, I have no idea. I don't see how a few empty lots could ruin the game.