User blog comment:Kirkburn/Technical Update: July 1, 2015/@comment-18172179-20150704230116/@comment-452-20150705050751

"So basically, this thing changes depending on the screen size?"

Yes, that's the entire point.

"I set my screen resolution to 1600px."

Excellent, we now have proof that users with 1600px screens get Desktop XL - thanks!