WhatsApp's backup to Google Drive doesn't work
The exact same happened to me, it would never finish uploading the backup to Google Drive and in every new attempt it would tell me that the backup size was bigger, start the upload and stop somewhere between 97% and 99%.

The only difference is this started happening spontaneously, it was successfully making and uploading backups every day and then one day, out of the sudden it couldn't finish uploading it.

After trying many of the suggestions that I found all over the Internet, like force stopping WhatsApp, clearing the cache, set it to use another Google account for backups, nothing helped.

At the end, the cause was the same issue pointed out in your last comment, I had a few corrupt 0 byte media files within WhatsApp's Media/ folder that couldn't be read, moved, copied or deleted: Operation not permitted.

I don't know how did that happen, but the best way I could get around it was to use a terminal App and run Linux commands for copying all non-corrupt files to a new folder:

cd WhatsApp
mkdir MediaCopies
cp -r Media/* MediaCopies


It will throw errors about the corrupt files but continue copying the good ones.

After checking MediaCopies/ looks right and contains all files then:

mv Media Media~
mv MediaCopies Media


Then open WhatsApp, poke around open pictures, videos, etc. and finally:

rm -rf Media~


It will fail to remove those corrupt files, but at least you won't have the rest of the files duplicated.

And voilà! No more issues uploading the backup.