Compression fields were a type of technology used to fit large objects into small spaces though miniaturisation. It was similar in use and capability with moleculon nanoscaler technology.
Miniscopes used compression fields to keep specimens inside them miniaturised. Specimens extracted from the field resumed their original size. (TV: Carnival of Monsters)
Members of the Slitheen family used compression fields in their collars to allow them to wear human skins and disguise themselves. However, this had the side effect of frequent gas expulsion. Initially, it was limited to humanoids of large builds. (TV: Aliens of London/World War Three) Compression field technology soon advanced to the point where they could fit inside the skins of thinner humans and life forms the size of footballs. Also the gas exchange issue was fixed. (TV: The Lost Boy, PROSE: The Monsters Inside)
One of the Slitheen's compression fields was acquired by Henry van Statten and kept in the Vault. (TV: Dalek)
The Teselecta used a compression field to miniaturise its crew to the size of an insect. (TV: Let's Kill Hitler)