Hi restif,

I'm not sure if I understand "deformation of the body part being smaller" right (obviously that's my poor english).
It sounds like your displacement is partially "carved" in the object instead of pushed out.

Possibly this might help you:
For displacement maps Carrara uses 50% grey as zero-level of the original surface.
Everything darker than 50% grey is lowered/ carved in the surface. Every lighter is raised/pushed out.