The previous thread was closed for some reason, so starting a new one.
I found about 30 Gbytes of files left in C:\ProgramData\Garmin\CoreService. This is in addition to the actual maps that are stored in C:\ProgramData\Garmin\Maps.
I understand the CoreService folders are just old downloads that are retained for some reason - just in case one might want to go back to older maps?
One simple fix is for Garmin Express to add a check box for "retain old downloads".
30 Gbytes is more than 10% of a 250G SSD, so it is not insignificant.