Subspace

From Egs Mayhem

Subspace is where some lifeforms can keep various items. Transformers often use it to store weapons, and some can use it to store extra mass when changing into a smaller form. This allows Rumble and Frenzy, who are both about 7' tall, to shrink to the size and mass of a micro-cassette, and Cerebros to increase in size when becoming Battleship Maximus's communications tower or Fortress Maximus's head.