This page demonstrates a 728×90 ad unit on Desktop and Responsive Ad unit on Mobile.
If you view this page on a smartphone, you will most likely see a 300×50 or 300×60 ad size, because that is the size ad, which Google typically shows on Mobile, if you use responsive ad unit.
In my testing, a larger ad, such as 250×250 square, outperforms Responsive ad by at least 50%. You should definitely test it on your site. You can use my free PHP Mobile Ad Switcher script.