@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.