Why the Linux total storage auto increase?

I had a Docker Image including a KVM and a windows.img of qcow2 format. And launched a container based on the Image, then I could connect to the windows by remote desktop connection.
Theoretically, running a container doesn’t take any storage, only when writing to windows can add storage in this case. But curiously, I did nothing in the windows, the total storage of Linux auto increase a lot (about 20GB) during the about four hours after windows booting up, and then the speed of the storage increasing slowed down.
What’s more, the windows.img file increased only 12GB, and the windows disk size didn’t change at all. After removing the container, the increased 20GB came back. I want to know why the total storage auto increased and where were the increased size.

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