This is because the VM has taken snapshots. When a snapshot is taken, the VM status will be recorded in the VM HDD image - including the VM data. Therefore, deleting the snapshot will not trim the size of the VM HDD image.
At this moment there are no convenient methods to reduce VM image size. However, you can use the “VM Clone” function to commit snapshots and reclaim the used capacity of snapshots. This function will delete all of the snapshots and the VM image size will be based on the actual usage of the VM.