Compact does not change the VDI's final size. It only dumps no-longer-used sectors out of the guest disk file on the host drive.
Your guest disk shows 76GB size with 36GB free. The guest disk file on the host drive is 67GB, and it can increase to the full 76GB. After compacting, your guest disk file should have been about 40GB (76 - 36) on the host drive. Did this happen?
If it did, then the compact command worked.
If it did not, please try Mpack's
CloneVDI. CloneVDI does not require 'sdelete' zero-filling, and has the extremely important benefit of making a compacted copy of the disk file, rather than working on the original and possibly bunging it up. I strongly recommend CloneVDI instead of the vboxmanage 'compact' command.
If your guest disk file did shrink to ~40GB, and you now want it to not increase past 46GB, you must change the partition size inside the XP guest. Shrink the guest OS's C drive partition to 46GB and the guest disk file on the host won't get bigger than 46GB.
Virtualbox does not come with a tool to edit guest OS partitions. XP also does not have a way to edit its partitions. The bootable 'Gparted' ISO can be used as a boot ISO on the XP guest's CD drive to edit the C drive partition to a smaller size. Web-search 'Gparted' for a download and instructions. Explaining Gparted usage is beyond the scope of the Virtualbox forum.
), which represent 2048 consecutive sectors of 512 bytes each. According to the documentation, CloneVDI uses its knowledge of the guest's file system to recognize unused clusters or sectors, thereby eliminating the need to fill those sectors with zeroes beforehand. But it still only eliminates completely unused or zeroed blocks when compacting, as VBoxManage modifymedium --compact does with completely zeroed blocks.