You might want to read: Publisher 2003 - What's new in web design for this
version:
http://msmvps.com/blogs/dbartosik/pages/80555.aspx
David Bartosik explains why the file sizes are larger in 2003.

From my experience, the biggest reason for file size bloat, and slow loading
pages, are the images, which can be mitigated. First of all go to Tools >
Options > Web tab and uncheck "enable incremental...", "rely on vml...",
"allow png...". Then in the best of all worlds, resize and optimize your
images for the web in a third party image editing program, before you insert
them into your Publisher document. Publisher will display a smaller version
of an image, but the load time is proportional to the original file size of
the image that you inserted. So, resize your images so that you can insert
them at 100% scale. (right click the image > format > size tab, and make
sure the height and width is at 100% under scale.)  In a less ideal world,
you can compress the images: Compress graphics file sizes to create smaller
Publisher Web pages:
http://office.microsoft.com/en-us/publisher/HA011266301033.aspx
If you reduce the size of your images correctly, the pages do load much
faster, even if the file size is a bit larger...and loading time is more
important than file size in my opinion.

From my experience thus far, Pub 2007 uses the same coding engine as 2003,
but at least the image compression is automatic. Otherwise, I am not sure
your file size will be reduced significantly from 2003 if you follow the
instructions above. Even with that as an issue, I still feel that Pub 2003
is a better version than 2002.

DavidF