@John.Locke's answer hits the nail on the head. +1 on your answer.
@DTest's comment confirmed it. +1 on your comment.
Here is how to get around it:
Step 01) Create an empty stopword list
$ echo -n > /var/lib/mysql/stopwords.txt
Optional : Make the stopword list have articles (no pun intended) 'a','an','the'
$ echo "a" > /var/lib/mysql/stopwords.txt
$ echo "an" >> /var/lib/mysql/stopwords.txt
$ echo "the" >> /var/lib/mysql/stopwords.txt
Step 02) Configure mysql to accept everything in FULLTEXT indexes minus your custom stopwords
[mysqld]
ft_min_word_len=1
ft_stopword_file=/var/lib/mysql/stopwords.txt
Step 03) Restart mysql
$ service mysql restart
Step 04) Reindex the table
mysql> ALTER TABLE articles DROP INDEX body;
mysql> ALTER TABLE articles ADD FULLTEXT (body);
Give it a Try !!!
CAVEAT
Anytime you reindex a FULLTEXT index after customzing to allow more words to be indexed, that resulting index with definitely be bigger.