Decrease body content width

I’m trying to make my posts more readable. At the moment I feel like they’re too ‘wide,’ and the line length (width) is just too long.

Here’s an example: http://aboveaverageshow.com/2015/12/32-confession/

In a perfect world, it would look more like the content here: http://thefederalist.com/2015/12/09/the-real-cost-of-raising-children-isnt-245000/

I tweaked some padding settings in my theme, but this changed the entire page (including the header) instead of just the post.

Suggestions? Thanks.