look on
http://www.activewidgets.com/grid/
I didn't see the source code yet, but I guess you're doing a string compare. For Integer values like Size and Date this gives wrong results. It will only compare the first position of each value, so the incorrect order for size is 1,339KB ; 101KB ; 2KB ; 3KB ; and so on. Compare integers as ($a == $b) and you are fine.