Using CopyToAsync file size problem

Hi,

I have an Air applicatiion in which I have to copy files from one drive to another.  I use spaceAvailable to get the space available on the target drive and run a script using src.size to get the size of the files being copied.  I would expect that the space available in the target location to be decreased by the src.size but it's always decreased by 20-30% more.  My concern is that I can't accurately predict whether there is enough room on the target drive.  Even if I copy one file the result is the same.

Does anyone know what I'm running into or a better way to check if the space available is sufficient for the files that need to be copied?

Thanks,

C.S.