I disabled all extensions, no luck. Then I even uninstalled Opera and reinstalled it, but still the same problem.
The rulers are correct. But somehow there might be a problem calculating the actual dimension.

I made a little test where I displayed the document.width using JavaScript. Thats the result:
https://vir2al.ch/o-ch.png

As far as I can see there are no zooms or anything else set.

Hy there
I'm using Opera 38.0.2220.29 for Windows 10 64-bit.
When I open the DevTools -> device mode I get very small dimensions for the mobile screen.
In Chrome the dimension look pretty similar to the result on my actual phone, on opera the screen is way too small:

https://vir2al.ch/opera_devtools.png

Does anybody know why the screens are so small?