Showing a smaller ad in an ad unit you defined might seem like a waste of resources at first. However, this is in fact a feature of AdSense called Similar-sized ads. This simply means that AdSense can show an ad (creative) that was originally set up by the advertiser to have another size than the ad space you defined on your site.
Simply, because it increases competition. The more ads can be potentially display on your site, the higher the competition and the higher the price you will actually get per click or impression.
The downsize of showing an originally smaller ad in a larger space is that it leaves space unused left and right to the centered creative. More about this below.
What many publishers don’t know is that also ads that were originally set up to be larger can fit into smaller ad units. E.g., AdSense can resize a Leaderboard (728×90) to fit into a Full Banner (468×60). AdSense does this for the same reason: increasing the competition for you to make more money.
Showing larger ads does not need more space. It will simply be resized to fit into the ad units size you defined.
You can simply switch off the similar-size feature in your AdSense account. But be warned: this switches off all resize features for all ads. There is no ad-specific setting for this.
Go to My Ads > Allow & Block Ads and click on the Ad serving tag.
In the Display ads section, you will find the Similar sized display ads option right at the top. I switched it off in the example on the screenshot, but it is on by default.
The question that can literally be worth millions is whether you should switch off similar-sizes or not.
I think you shouldn’t. Having only ads displayed that perfectly fit into your ad units might cut the competition to only a fraction and decrease your ad revenue from AdSense a lot.
It might be a smart idea to just collape the space left unused if a smaller ad is displayed. However, due to the method AdSense uses to deliver ads and how it predefines the space it is not possible to automatically collapse the size.
Instead, if you want an ad that only takes the available space, take a look at the Responsive ad units from AdSense. They have their flaws too, though.
Let’s just assume there is a way to collapse the unused space automatically in the near future. This might be irritating, too, because it would cause the already loaded content around the ad to move by the collapsed space again.
If you like the AdSense performance reports as much as I do, you will be interested in this: Asside from the normal Ad sizes report, there is one called Creative sizes, too. This report contains the sizes of the actually delivered ad sizes, not the ad unit sizes which you defined.
You can combine your ad specific report with the Creative report to see the ad sizes actually delivered to a specific ad.
Use this to see if there are really many smaller or larger ads delivered. If you still think there are too many “wrong” ad sizes used, you can switch your ad units to another size based on this report without losing too much of its performance.