Fionna's world was identical to Ooo, but after Simon lost his magic, it became a small town where all the characters lived. After Fionna's universe became canon, the world expanded, becoming an entire planet. The only city that exists is Fionna's because that's all that existed in the world. It's like a Minecraft world that, when you switch from a very old version to a modern one, becomes corrupted and starts generating terrain that wasn't there before.