You didn't post enough info to know if it was a FAT problem or not, but FAT or a tool that can't deal with files over 4 gig in size are the common problems for the hugh size discrepency that you reported. There is also a well known problem of the mirrors way too frequently corrupting ISO downloads; I had about 50% bad downloads (md5 sum wrong and the file slightly wrong in size) before Knoppix started becoming available by BitTorrent. Since BitTorrent has been available I've not had a bad download and have downloaded much faster than I could before. But the corruption issue would not explain the hugh file size difference that you wrote of, so I didn't focus on that.