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Maybe you are right, universes are pretty small. That's why our universe is the "fancy box". Maybe anyone who doesn't create and enter their own box simply resides within the box of the being currently holding the position of Universe.
Oh and there was a concrete reason for this song, "I love you universe": the girl on the left is our universe, or the past Universe, while the girl on the right with yellow hair is the new Universe, these are correct representations within this artistic style.