[Fyron]

Size is the exact file size. Size on disk is essentially how many total clusters (or maybe it is some other smaller unit, I forget) the files take up on the hard disk. Every file has to take up space in specific sized chunks on the hard disk. The size of these chunks depends on how the disk was formatted. The size of the file should be the same when it is stored on any PC, CD, disk, etc. The size on disk is the total amount of space used up by all of those chunks. It will always be a bit larger than the actual file size, as it is very rare for a file to fit exactly into X chunks of hard disk space.