Here is what the spec authors have to say on this:
Currently Hub seems to be sending entire feed and this is a scalability and usability issue.
In a discussion with Robert Cooper this is what he had to say:
"The big problem is that the notification from the publisher doesn't contain new data, just a request to re-fetch. What we really need is a new implementation of a FetcherCache that returns a SyndFeed that only contains data that is new since the last time it was fetched. The question is, how to determine what is new... I would tend to say we need a synthetic key based on [post ID | ->link URL] & [publishedDate | editDate]."
Fixed in r80.
In order for Hub to deliver Notifications to subscriber that only have changed entities it MUST use a FeedFetcher that uses a DeltaFeedInfoCache. Otherwise the hub continues to send entire feed to subscribers.
See DeltaSyndFeedInfoTest on how to to Hub setup with new DeltaFeedInfoCache.