Actually, I believe OS X does this for small files in order to not burn a whole filesystem block on a sub 1k file.
"In Mac OS X Snow Leopard 10.6, HFS+ compression was added. In open source and some other areas this is referred to as AppleFSCompression. Compressed data may be stored in either an extended attribute or the resource fork.[13] When using non-Apple APIs, AppleFSCompression is not always completely transparent."