Many of you have been noticing that you can have an AdSense ad that is a large box, and have a small little ad in it. In other words, they are sometimes showing 4 ads, some times 3, some times 2 and some times 1. The less ads shown on the page, the more white space (depending on your AdSense background settings) is shown. The more white space, the less professional your site is to your visitor.
A thread at WebmasterWorld Forums named 336x250 only showing two ads? discusses just that. On occasion Google will increase the font size of the text in the ad, but not always. When it doesn't some folks feel as follows;
Its just that it makes the site look funny with all this extra whitespace. Kind of makes it look as if the site owner can't sell the empty ad space of something.
Forum discussion at WebmasterWorld.