Google announced a new change to ad serving with in Google AdSense. Now, if Google finds a better performing ad type, even if it is smaller than the ad slot you currently set up, Google will place the ad in that ad slot and place white space around it.
This is called, "Similar Sized Display Ads" where Google will try to throw in better performing ads even if they don't look as good.
It may look like this, with white space around the ad box:
Google explained on the opt out page, under Allow & Block Ads > Ad Serving:
To help increase competition for some ad sizes, we occasionally show similar-sized display ads within larger ad units. When this happens, the smaller ad is served neatly in the center of the larger ad unit. For example, you might see the following:
- a 160x600 ad in a 300x600 ad unit
- a 728x90 ad in a 970x90 ad unit
- a 300x250 ad in a 336x280 ad unit.
Note that this is not the complete list of ad units that may show smaller ads and the list is subject to change.
Here is a screen shot on how to turn this off:
Seems like most AdSense publishers are happy with this new change.
Forum discussion at WebmasterWorld.