When you see a text ad that has expanded to take up more of an ad unit or the entire ad unit, you may be seeing either a placement-targeted or a keyword-targeted text ad.
Placement-targeted text ads appear when advertisers select specific sites on which to run their ads, on a pay-per-impression basis. Their ads are then expanded to fill an entire ad unit, making placement-targeted ads even more appealing for advertisers, and resulting in higher bids, meaning higher earnings for our publishers.
Keyword-targeted text ads may also be expanded when we determine that a larger ad will perform better for a particular page. Our AdSense technology will automatically determine the optimal number of ads to display and will only show fewer ads when it improves monetization.
At this time, it isn't possible to opt out of displaying expanded text ads. The number of ads shown in your ad units is automatically determined in order to maximize clickthroughs and revenue.