Update:
If the "display scaling" in Windows is normal (100%), the problem doesn't occur on high-res displays.

However on high-res displays, you need to set the scaling at 150 or 200% to see anything properly, and that's when the checkbox / radio elements DO NOT scale up properly in Opera.

Basically checkboxes and radio buttons do not work.

Example: https://www.zoho.com/projects/login.html
checkbox next to "Keep me signed in"

Also applies to radio buttons.

With checkboxes, I also can't see if they are checked or not (even if "zoomed in" all the way).