I opened a new tab and rested my fingers on the trackpad. Must have done something, because all of a sudden it zoomed in A LOT. When I zoomed back out everything was really small as if it was optimized for a monitor with a much larger resolution. The text is tiny and the layout is slightly different. Restarting Opera didn't solve it.
I use Yosemite 10.10
Opera version 43.0, but I updated after the glitch (45.0)
I have a screenshot but I don't see a way to put it in my post.
Use the menu View -> Actual Size to reset the zoom level.
You can also use Cmd-0 to get the actual size.