I do my backup to the cloud (iCloud). I always opt to include media. I noticed that the size of the back up is slightly smaller (557MB vs 577MB) than my RM media folder.
How can that be? Everything media item I enter into RM is dragged and dropped from the media folder.
The backup compresses the data, so the backup will be slightly smaller than the un-compressed media files. (If I follow what you are saying)
You are lucky to have such small backup — my RM file is over 220 mb without any media (other than the thumbnails inside db). The media folder is over 15GB -which is why I almost never do full-backup. I am not sure of the compression method but you are likely to seem some savings in file size.
Thank you, that makes perfect sense.
It’s only that small because I’ve barely started transferring my data 
RM uses a zip compression mode (there is more than one). Renaming the.rmbackup file to the .zip extension allows any zip utility to open the file and extract the compressed contents.
If any media file in the common folder becomes detached (unlinked) from the database, it is not included in the RM backup. For a valid comparison of folder size and backup size, all the media files and the database file must be in the same folder and the media files must be only the ones attached to the database.