The justly popular Feedburner service comes with a 500-kb limit. Exceed that and Feedburner will not syndicate all your content, dropping the oldest first.
This is consistent with the model of a feed as a broadcast of your posts to subscribers (via feed reader or email). Such readers presumably are only interested in your latest and greatest.
However, complete feeds that comprise all of your content are broadly useful for other purposes, such as my Blog Journey hack to show posts in chronological order, oldest first.