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If you wait two days, WebODM will clean up temp files.
However, note that the ext4.vhdx that WSL2/Docker for Windows uses is a dynamically expanding volume, meaning that its minimum size on disk becomes the largest size it was ever allocated for.
So, to re-shrink it, you will need to use a tool like the Windows Hyper-V Management tools to shrink the disk after WSL2 has been taken offline.