when backing up a virtual machine via the web interface, the backup weighs as much as the entire VM disk

[xczbvmzBNvyurG]

only 1/4 is actually occupied, everything worked fine before and they weighed several times less

if you look inside the VM, almost as much is occupied as before the problem appeared

Disk type: qcow2, discard enabled

 

[fba]

The underlying storage needs to support the "Discard", like Ceph, ZFS, LVM Thin do. The OS of the vm needs to do the trim nevertheless too. So in Windows it is the optimization of a drive, in Linux just run fstrim -a.
More details can be found here: https://pve.proxmox.com/wiki/Shrink_Qcow2_Disk_Files

 

[gfngfn256]

only 1/4 is actually occupied

To realistically check that use:

qemu-img info path/to/file/filename.qcow2

Except for running a trim within the guest on that virtual disk, you should also use some form of compression for the backup, Proxmox uses ZSTD by default.