Filesize Difference

From windows I've copied a number of files. They are of .tga and .psd files, but when I transfer those file in my thumbdrive over to mac pc.. I found a difference in total filesize. What are the possible cause of this event??

OS X Snow Leopard is by default using base 10 to calculate file size, windows is using base 2. OS X switched to showing drive space in base 10 because consumers were all confused as to why their "160GB" hard drives were really only 147GB.

Strange, actually, the file size become smaller when i copy from Mac to PC. And when its from PC to Mac, the file size remain the same... But then when .mov files are transfer over instead the size remain the same for both mac to pc and pc to mac.. Any idea why?

NOTE: Thread moved to appropriate forum. Not just anything goes in Anything Goes