When used as designed, FTM will put a copy of the "media" in the "Media Folder" that is associated with the FTM data file, and a link to the copy.
One MAY chose to have the selected media remain where it is and FTM will provide a like to that location.
BUT, when making a full backup of the FTM data file, the media that is NOT within this "Media" folder will not get included in the backup file.
I do agree with KathyMarieAnn, that linking the FTM data file should not get any larger, except for the required "1's and 0's" to define the link.
I too, do NOT use many "images" or other media in the FTM data files.
See the attached image, the "Fike-Napier.ftm" is the FTM data file. The Fike-Napier media" folder contains 10 images of persons used for the 'portrait' images. This is a very small family that I have had for some time and use it a lot for testing different parts of FTM. I have NOT seen any increase in file size when I added the images for the portraits.