Since the VHDX is dynamically expanding, it grows when you add files but does not automatically shrink when you delete files. If you deleted large files inside WSL, the ext4.vhdx file will still occupy that space on your Windows drive.
To view or edit your Linux files directly through Windows File Explorer without finding the hidden .vhdx file, use the following methods: Location of WSL $Home directory (in Windows) - Super User wsl disk location