How much extra storage does MySQL fulltext index consume?

I am looking for a way to estimate how much extra storage will be needed if a MySQL TEXT column gets a fulltext index.

Intuitively speaking the size of the fulltext index is depende开发者_StackOverflownt on the total length of text but what exactly is affecting it?

"I have 200 MB of text data to index and the resulting index is 130 MB."

So it's comparable to the total size of data you index.