Why does vhdx file take more space than actual data in ext4 partition

I know Hyper-V is not able to handle ext4 file system, thus the compact will not compact correctly.

But I found some articles on the web suggesting zeroing unused space.

df -h actually returns :

Sys. de fichiers Taille Utilisé Dispo Uti% Monté sur
udev               424M       0  424M   0% /dev
tmpfs               88M    488K   88M   1% /run
/dev/sda1           20G    1,9G   17G  10% /
tmpfs              440M       0  440M   0% /dev/shm
tmpfs              5,0M       0  5,0M   0% /run/lock
tmpfs               88M       0   88M   0% /run/user/1000

Which tends to state that only less than 2Gb is used.

But vhdx file was 5,8Gb.

I used fstrim command in Linux VM (Debian 11), then shutdown the VM, and started to shrink the vhdx. It went to 4,7Gb. Still more than double as the content.

Then I did :

dd if=/dev/zero of=~/zeroes
sudo sync
rm ~/zeroes

Then shutdown and shrink the vhdx, no change, still 4,7Gb.

What's happening there and how is it possible to recover more space ? (I have a lot of Linux VM's with same issue, taking up a lot of unnecessary space on my test host).