I'm a big fan of Graphic Converter. It's not the friendliest app in the world, but it sure does a good job of compressing Jpegs.
As you probably have figured out, I add a second step to iPhoto's slick web page generator. In addition to adding a little custom HTML at the top of the page so it isn't so danged dull looking, I use GC to further compress the pictures in the "Images" folder.
If you've used iPhoto's web generator, then you know that it creates 3 directories and an "index.html" file. You don't have to mess at all with the "Pages" directory. And happily, you don't have to play with the thumbs in the "Thumbnails" directory because iPhoto is good at compressing them. But I do recommend that you reduce the default setting for the thumbs from 240 pixels wide to a more reasonable 180 pixels wide.
But I think iPhoto is a little sloppy on compressing the pictures in the "Images" folder. I recommend that once your web directories are created, open those pictures in the "Images" folder with GC, save them between 70 and 80 "Quality," and make sure that the "save web ready" box is checked.
By doing this, you can reduce the file size of your 640 x 480 "enlarged" images by about 50 percent of what iPhoto initially creates.
My guess is that future versions of iPhoto will give us better compression control.