If you move the mouse cursor all the way left (or right), the pointer changes to a horizontal-resize indicator. At this point, you can click and drag toward the center, to change the width of the page while the browser stays in full-screen mode. A gray linen background appears on the left and right of the resulting narrower page.
Each tab can be adjusted independently.
[crarko adds: I tested this, and it works as described. I tried this in Preview and it seemed to resize the page automatically when switched to full-screen mode.]