How to convert a large dynamic virtualbox VDI image to a small img file?

I have a VirtualBox dynamic image of Xubuntu 20.04 of maximum size 70 GB. The current size of the image is 9 GB.

When I try to convert it to a .img file with the command
VBoxManage clonehd 'xubuntu.vdi' xubuntu.img --format RAW

the image size keeps growing far beyond 9 GB, and it would probably go on until 70 GB.

How to turn it into an image of size 9 GB?

The host OS is Kubuntu 20.04.