I'm pretty sure this is because your iMac has a MUCH greater resolution screen than your macbook, so when you make the browser window larger on your macbook it still is resizing the displayed content to a larger size because you're still within the maximum page width that that particular website's programmers set their content to display at, but when you do the same on the iMac you've already passed the width set by that site's programmers. I know I didn't explain that very well. If I'm right, then you'll see that only *some* web pages are cramped into a narrow strip of your browser, while other web pages will fill the whole window with the page's contents. Have you noticed this happening?