RSS/Atom kept it simple (and for the most part, kept it small). But Holy Hijack! News feeds (like the Big-G's) are pushing the limits by embedding oodles of HTML (table, divs, images, links, ...) ... blurring the boundary between RSS reader and browser. As an example, the Sci/Tech news is 150+K, while the RSS feed is a little over 90K ... for little more than the teaser text on just 10 stories? WTF?!
Obligatory AppEngine blurb: on the first day, the Big-G received registrations from its requisite 10,000 beta developers; other applicants are now being on put on a wait list.