surpassing database limit because of fulltext index FTS_[…]_INDEX_1.ibd

Dear WordPress-Community,

I am running a blog for three months now. Two days ago I received an e-mail from my webhoster who informed me that my database has surpassed my limit of 100MB/database. The reason seems to be a full text index. My database contains two files FTS_xxxxxxxxxxxxxxxx_xxxxxxxxxxxxxxxx_INDEX_1.ibd and FTS_xxxxxxxxxxxxxxxx_xxxxxxxxxxxxxxxx_INDEX_1.ibd (second xxxx number differing), both with a size of 40MB.

Now I am wondering about the cause of this extraordinary size. The only thing I was able to find on the net was this post on stackexchange

This does not give me any hints what part of my wordpress blog might cause this. Does anyone have a clue where these indexes come from, by what they are used and what possibilities I have to decrease their size sustainably?

Notes:

* I use WP 4.2.2 with Parabola Theme and currently have an amount of 17 posts and 4 sketches. The blog may be found here

* My plugins are: Add Meta Tags, Antispam Bee, Better Delete Revision, Broken Link Checker, Cachify, Contact Form by BestWebSoft, Contextual Related Posts, Explanatory Dictionary, Google XML Sitemaps, Raw HTML, Simple Share Buttons Adder, Subscribe to Comments Reloaded, Top 10, UpdraftPlus, WP-Piwik, WP-Benutzer-Avatar, ZipList Recipe

* I am familiar with computers, but have never looked into databases.

* My host offers phpMyAdmin to manage databases. I’m not able to find these indexes there. My host is currently looking into this.

* Please tell me if any further information can help you helping me.