It assumes that "empty" flash memory is all zero-bits, while "used" memory is a mixture of ones and zeros.
I believe that's not true with the FAT filesystem (which most/all readers use), empty memory can contain anything, it's just marked as empty, or rather not marked as used, in the file allocation table (FAT). So I'd say "in principle, maybe", it might gain weight, it might lose it, but it will remain FAT