I was noticing my iWeb blog was a whacking 18MB for about six entries! I do not use .Mac, so I can't say if this works for those who do, but I publish my iWeb to a folder, and then upload it via FTP.
Anyway, when looking in the folders that contained my blog (they are in a folder with the the same name as your blog), iWeb had exported the originals of the photos I added, along with the reduced size version used on the page. These full-size images can just be deleted and the site works perfectly. Also, as this is an exported version of the site, the copy that opens in iWeb for editing is unaffected.
The ones to delete are easily identifiable, as they're all called myphoto_1.jpg, where the one iWeb uses are just called myphoto.jpg. Looking in each blog page (search through all the folders), I reduced the blog pages from its original 18MB down to 2MB! (All my originals were from a 7MP camera, so are each about 3MB).
Interestingly enough, iWeb does not store the original versions of photos you put on any other type of page: photos, homepage, etc. Gotta love the consistency of this program. Bring on iWeb 2.0!
[robg adds: I confirmed that this is the case with an exported site's blog page: each image's full-size original is included.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061119094438443