On or around line 1043 of 2014's style.css you'll find several rules that set a maximum width for content. You might try removing those max-width settings.
I'd also add that you will be wise to make your css changes in either a child theme (check the codex child theme page) or by use of a css plugin, rather than to the theme's style.css. That way when the theme gets updated you won't lose your changes.