>>151001565
>I'm not convinced it's an entire world either. Fionna's city seems isolated and dreamlike.
Am I the only one who didn't have a problem understanding this? Pic related was the before, they literally showed this before they showed us how it got filled out at the end, and they even showed how it expanded AGAIN in the new season >>151001881
>Did a bunch of people and places spontaneously come into existence when the world was canonized? Were the people in the city given memories of the world beyond?
You're way overthinking this, the simplest explanation I assume is that there simply was an implied world outside the city, but they just never left, and now that implied world outside the city actually exists and they can leave if they want, that's it.

>>150873144
We literally saw it go from pic related to being completely full.
I would assume the implication being that it's as big and expansive as every other world.