On the iPhone version of Safari, it’s designed to show Web sites that are made for your desktop.
If you tell the CSS to make the width of your site only 320 pixels, it will look all zoomed out, like as if you were viewing a Web site that was 1200 pixels wide.
You have to constrain the viewport in a meta tag. Like…
<meta name=”viewport” content=”width=320″ />
With this, your site will only take up the 320 pixels of width that are available on the iPhone. Check out my iPhone screen size test page.