I've been backing up my WordPress database from MySQL using MYSQLDUMP - I have no problems with that, I'm happy to say both backup and restore are going well.
However, when I examine the SQL data in the backup, I can see that there are huge chunks of data under wp_options #66 (rss_0ff4b43bd116a9d8720d689c80e7dfd4) and #68 (rss_867bd5c64f85878d03a060509cd2f92c) - the latter being the largest. These appear to be entries from the WordPress Development Blog (#66) and entries from random blogs I have never heard of, some of which mention WordPress, some not, over 100KB worth.
This data is not visible to anyone accessing my site, as far as I can tell. I don't even see all of it on my Dashboard, since all the links are different and take you straight to the servers that own the blogs in question, not to data cached locally.
I tried deleting those records, which cut the size of my SQL backup in half (230KB ~ 112KB), which did not affect the operation of WordPress, but it was there next time I added a blog entry. So: why is WordPress cacheing this data in the MySQL database on my server, under wp_options, with no clear label?