Compression fields were a type of technology used to fit large objects into small spaces.
Miniscopes used compression fields to keep specimens inside them miniaturised. Specimens extracted from the field resumed their original size. (DW: 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 large people. (DW: Aliens of London / World War Three) Compression field technology soon advanced to the point where they could fit inside the skins of thinner humans. (SJA: The Lost Boy)
One of the Slitheen's compression fields was acquired by Henry van Statten and kept in the Vault. (DW: Dalek)
The Teselecta used a compression field to miniaturise its crew. (DW: Let's Kill Hitler)
The Abzorbaloff used similar technology in the form of a limitation field which compressed him and his victims after he absorbed them to keep the absorption ability from going out of control. This also caused gas exchange. (DW: Love & Monsters, REF: Doctor Who: The Encyclopedia)