How to make AdSense responsive ads show rectangle ads instead of horizontal ones?

I noticed that many sites with AdSense responsive ads show large rectangle ads (336px x 280px) or large rectangle text ads (full width x 280px). However, in my own website (640px width content area), it shows most of time horizontal ones.

How to make AdSense responsive ads show rectangle ads instead of horizontal ones?

At SysTutorials, we use responsive AdSense ads and also prefer the rectangle ads. With some tunning, you can control the style of the adsense ads as shown in the follwing screenshots.

Text lists:

Display:

Big text:

The key is to change

data-ad-format="auto"

of your ad code to

data-ad-format="rectangle"

You do not need a premium adsense account to do this. It is just an advanced adaense feature: https://support.google.com/adsense/answer/3543893#adv .