Subspace compression was a force of nature that affected subspace.
While attempting to decrease the internal mass of the two pieces of the asteroidal moon threatening Bre'el IV with the warp field and tractor beam of the USS Enterprise-D in 2366, Q warned Enterprise Chief Engineer Geordi La Forge that if his attempt to use the field symmetry to compensate for the different inertial densities of both pieces of the moon failed, the moon would "crumble due to subspace compression". (TNG: "Deja Q")
The subspace compression anomaly discovered by Starfleet in 2374 created severe spatial distortion, including miniaturization. (DS9: "One Little Ship")