1. When the pump is on, something is expanding in volume a little. NORMAL for plastics and tubing.
2. Water is not compressible (let's accept that for a moment) but there is some note from you that it's almost bubble free so any bubble can be compressed.
Are you fretting over this a bit too much? I mean, does it work?
I am trying water cooling for the first time and something weird is happening. My loop is working fine and I am using a T-line to fill it. It is full and almost bubble free however when I turn the pump off the water level rises a little bit in the filling tube. Does anyone know why the water level might rise when I turn the pump off?