Suppose I have a feed with a million items already on it which I've already handled. I'm about to poll the feed and there's a small number of new items on the feed I don't know about yet.

Am I going to have to download a file with references to all one million items again?

If the answer is to have a separate feed with only the most recent n items, I'm afraid that's not going to work unless there's additional details. There might have been n+1 items turn up since the last time I polled. Also, someone else might want to start handling those one million records from the start and we should be able to co-ordinate by having a common canonical URL.

(I'd also mention pull-vs-push, but that's a thread elsewhere in these comments.)