Subspace Storage is a type of technology used to create artificial dimensions.
Used by both the Protoss and Xel'Naga, subspace technology shunts large quantities of objects in to very small spaces. Often applied to structures and vessels, a city's worth of personnel can be sustained in a single Mothership. Some earlier versions of Scouts have been known to be modified for sustaining a single pilot or skeleton crew for long term missions (Dr. Who: TARDIS; Star Trek - Enterprise: Time travel pod).
Zealots and some worker castes possess personal versions in the form of belt attachments to hold different types of equipment (Legend of Zelda: Pouch and Cartoon). However, these attachments are limited in capacity due to power constraints.