There are utilities to compress HTML, but I cannot gaurantee the code will load back into iWeb for editing later on.
http://www.tucows.com/preview/219162
If not that one, type in "compress HTML" in Google and look around. I have to say I am behind a firewall at work, those sites are restricted so I cannot suggest a "best" choice. Other responders might chime in, of course.
What I would do is concentrate on optimizing images and removing comments from CSS and Source that don't need to be there. That helps alot, and simple to do. in iWeb you'd export the site, then edit by hand, then upload.
Another approach is to enable GZIP compression on your server
if you host the site on your own server with Apache. There is no way I am aware of to do it via code via iWeb, much like PHP has GZIP capability. So this is a question for your private webhost. For the record, GZIP is not all its cracked up to be as there are different levels to choose from and traditionally its controlled by the developer in their code, not the server.
-jim