Extra Margin When Switching Between Different Sized Computers

Hello –

I am designing a site with Nirvana and I am having a problem with the margins. On my laptop, the margins are pushed horizontally to almost each side of the page as normal, but if I view the site on a larger computer (like a big desktop Mac), huge margins appear on the left and right sides of the content. This might be a bug or an issue with how it’s responding to different sizes of computers. I can’t figure it out. Thanks!