Did you mean to say the slow one has a lot more? Regardless, I think there’s no verbosity that gives per-file information on changed data, however there is a way to see what files are being added, deleted, or changed.
Sample output:
Listing changes
1: 11/20/2018 8:41:59 AM
0: 11/21/2018 10:02:18 AM
Size of backup 1: 343.77 MB
1 added entries:
+ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\bookmarkbackups\bookmarks-2018-11-21_1422_1kdKfrSWi29YPJOva1+VJg==.jsonlz4
5 modified entries:
~ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\bookmarkbackups\
~ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\sessionstore-backups\
~ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\sessionstore-backups\previous.jsonlz4
~ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\sessionstore-backups\recovery.baklz4
~ C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\sessionstore-backups\recovery.jsonlz4
1 deleted entries:
- C:\Users\Ted\AppData\Roaming\Mozilla\Firefox\Profiles\rsbrp398.default-1528892271354\bookmarkbackups\bookmarks-2018-11-20_1422_1kdKfrSWi29YPJOva1+VJg==.jsonlz4
Added files: 1
Deleted files: 1
Modified folders: 2
Modified files: 3
Size of backup 0: 343.67 MB
and then you must guess which files were the offenders in making a big upload. Note that the backup is on a file-block basis, and only changed blocks are uploaded. Unchanged blocks from old backups are referenced.
EDIT: A nicer web-UI-Commandline version from @kees-z (thanks!) of running a compare just went up here, for someone wanting basically the same thing (or at least giving them something that comes kind of close…).