windows vm size continually growing after opened but not back to original size after shut down?

Hi,
I am using parallels 14 and installed windows x64 vm on macbook pro with os mojave. I found windows vm size growing after opened each time. For example, today is 33 gb, tomorrow is 33.5 gb and next day is 34 gb but I did not install any programs, did not do windows update or download any data during this periods.

The windows vm size keep increasing daily after opened and never reduced again.
I understand that open vm may increase disk size but I thought it should back to original if vm shut down. Does anyone have the same situation or I have any misunderstanding?