UTM does not support resizing disk images formatted with APFS.
You actually have a complicated situation here. With APFS disk images you have to increase the size of the disk image and then increase the size of the APFS container in the image .. just like you would with a real formatted disk .. however, your situation is complicated by how macOS formats a APFS boot volume. Your image will have multiple APFS containers in a Volume Group -- Macintosh HD (system), Macintosh HD (system snapshot), Macintosh HD - Data, and Recovery. What you want is to increase the size of the disk image so you can increase the size of the Data container w/out impacting the other containers, which may not be possible depending on how the container locations are assigned.
The easiest and most straight forward option is to back up any data in the current image, create a new, larger image, reinstall macOS in the larger image, and migrate your backed up data to the new image. The larger image will have a larger Data container.
Is there a way how to resize an existing disk (the primary one) of an macOS VM? It defaults to 64GB and thanks to APFS it occupies only the real size of data within (via du -sh on the disk image) e...
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