Abnormally high MySQL writes and larger than normal Binary Log files. How can I determine what caused this?
Don't use --short-form.  You are suppressing much of what you want to see.

--short-form
Display only the statements contained in the log, without any extra information or row-based events. This is for testing only, and should not be used in production systems.
— http://dev.mysql.com/doc/refman/5.6/en/mysqlbinlog.html#option_mysqlbinlog_short-form

That is a largely pointless option, since it returns only the statements contained in the log.  If you have a binlog that large, it's unlikely that it's full of statements -- it's full of row events.
--base64-output=decode-rows --verbose are the options you are looking for.
http://dev.mysql.com/doc/refman/5.6/en/mysqlbinlog-row-events.html
You will, at the very least, not be disappointed by the quantity of data returned... it will be much larger than the actual binlogs themselves, which are stored in a relatively tightly-packed format.