Downloaded file having different sizes and corrupted

[pandane]

I'm using a DS411j unit with latest DSM4.0 firmware. I recently noticed when I downloaded a large number of pictures wirelessly to my local computer, there will be a few corrupted pictures which are unable to be opened. However when I download these files from NAS again, they will be fine. When I compared the corrupted files and originals, they seems to have different file sizes, like 2.23mb vs 2.22mb. Does anyone have similar experiences and any idea if the problem may be due to the NAS, the wireless router or my computer? Thanks!