doesn't it have to do something with cluster size? if you have large files you're better off with large clusters (for speed), but small files take up a whole cluster so if you have lots of small files you need a small cluster size, because a file takes up at least 1 cluster, not half a cluster.
so if you have 4kb clusters and you have a 50 byte file, it will say 'size on disk: 4kb'.

but i don't think you can change cluster sizes on a memory stick, can you ?