You can compress your backup from the context menu in the backup archive to save some space.
After making the selection you get a confirmation.
After selecting Yes the process starts.
As this is done by the worker process you do not have to keep this screen open, you can just close the app and walk away. However if you keep the screen open you get to see a progress bar as long as it is working.
Note that at this moment in time, the compress progress bar sadly is not a true indicator on how far your archive compress is towards being finished, it merely indicates progress. The reason being that the native macos zip command did not give us the correct information on that to show you this correctly.
Once the compressing is done it will show you that like this: