Groupwise 8.0.1 HP1

Why is there such a difference in the size between a message/attachment when it is within Groupwise as compared to when the attachment is saved locally? I realize some of the difference is because of the message itself and some is just simple GW overhead but.... Example: A user has a message/attachment that shows in GW as being 1.5M (with very little text in the message body), but when he saves the attachment onto his C: drive, it is only 65K. That's a big difference.