Volume/Surface of a deformed medium

I used a simple 2-D geometry of a solid material.
I used "solid mechanics" module and applied different external pressures to the material.
As expected the material was deformed under ther external pressure.

But .....
I now I try to calculated the changed volume of material.
An interpretation in the postprocessing indicates constant surface of the material.
I used "Results/DeriveValues/line integration" and applied it to the expression "1". I expected the integral to be the length of the way = the surface of the body.
But it result sticks to the original length without pressure, independend of the applied external pressure.

Any ideas?