Thanks for bringing this up! We actually do federate full posts as Articles instead of Notes – Mastodon actually converts our posts to Notes, and only displays the title and URL. Today there isn’t much support for this ActivityStream type in the fediverse today outside of Pleroma, WriteFreely, Plume, and a few others.
We have started experimenting with sending out full-text Notes here: writefreely#220 – it’s based on line breaks, rather than text length. But I’m sure there are plenty of ways we could do it. Would love to hear your input there!