Okay, I think I figured it out.
Although these other website do have wrapper divs which far exceed the width of most devices, the actual content is inside a div which is less than 1080px wide.
My Windows 8 tablet has a resolution of 1920x1200. So when I rotate to view in portrait mode (display width of 1200px), it appears as if the website has shrunk (or been zoomed out of) to fit, but its not. Its just the extra background detail in the wrapper div has been chopped off, but the content still fits in the portrait width of my device which is 1080px.
If my tablet had a lower native resolution say 1024x768, then even a 960px width website would get chopped off in portrait mode.
Finally, and maybe most importantly, any divs that provide a background must not have a width set that is not flexible. Basically, always set CSS attribute to max-width: 1200px;
rather than width: 1200px
. This means that on smaller screens it will collapse to the width of the inner content div.