Jeff,
Two things to note here. If you're using the full message body as a shortcut, the actual size of the shortcut will be close to (or about 70%) of the mail message itself (without the attachment). If you want to see the size reduction from shortcutting, send yourself a 3mb (or anything large) attachment, and then archive that item. it should go from 3bm to like 5k.
Second thing, exchange deals with mailbox size reporting in a very weird way. The only way to see an actual reduction in the mailstore db file size is to do an offline defrag (which is time consuming and takes down the mailstore). When EV archives something, it leaves whitespace (which exchange will then reclaim and use for other incoming mail), but doesn't resize the mailstore dynamically.
micah