Change width of content area

I want to increase the content area, so I tried this:

.content-area{
    width:1280px;
}

That did increase it, but the main content is pushed to the right, and to much space on the left.
A little strange I think, since the container have left and right margins set to auto.

So how do I accomplish this the right way?

I wish to only modify the main content, and not the header or footer.