It's reasonable to have whole articles in RSS, of you aren't trying to show ads or similar.
100 articles are not reasonable.
100 articles where most of them are 1+ year old is madness.
RSS is not an archive of the entire website.
Whole-article feeds end up become exactly that - a local archive of a blog.
The host is fine with sending 0.5 MiB once (the client should be aswell from both a bandwidth and storage point of view).
The host is not fine with sending 0.5 MiB every 20 minutes, which could be easily avoided if the client would use the mentioned "If-Modified-Since header".