Because by default, the series (0,1,2,4,8,16) are Category labels rather than values in Excel, so no matter how the big the number is, they will be equally spread.
What you want to do is to make the X-axis linear, so instead of considering the series as category labels, they will be seen as real numbers.
In the link below, I made a chart with FunFun playground, you could see the labels X-axis are spreading linearly according to their values.
You could also link this result directly to your Excel by URL. Here are some screenshots of how it looks like in Excel.
Disclosure: I'm a developer of Funfun