That post for 5 is two years old and refers to the old interest rate system. So its misleading to say that it doubles every year now. But inflation rates are now in the order of 9% and that mostly goes to curators and witnesses . This post has a better explanation of the interest rate and its about 1.3 to 1.9% . https://steemit.com/steemit/@preparedwombat/yes-you-do-earn-interest-on-steem-power-all-well-most-is-revealed