to compress a STEP file you can use FreeCAD >= 0.18
FreeCAD can open and save .stpz compressed STEP file format.
KiCAD doesn’t have direct support for .stpz file format ATM; it will be available in version 6 later on.
ATM you can steel use a STEP compressed models through StepUp.. just associate the VRML model to a kicad board and convert it with kSU.
Mainly it is a useful feature for saving disk usage…
@kammutierspule is there a way to delete the 3d cache? I think this should be addressed, considering that kicad has different versions of 3D libraries