I've not seen this either, all manual and automatic backups contain the thumbnails are are roughly around the same size. I don't use the scheduler to make library backups though... so badger may be onto something here, if backups are done with the scheduler maybe they don't backup thumbnails since support for that hasn't been added yet? If that's the case, that's something that may need to be supported/addressed.
That's something Matt could probably answer for sure.
