I would try turning off all the blocks you have enabled on the front page first and see if that returns you to 100% page width. If that works then its likely one of your blocks isn't wrapping properly or stretches beyond its confined space for some reason.

Should the page revert back to 100% after disabling all the blocks, enable them one at a time until you page stretches to its current size. When that happens, you'll know what is causing your problems.