Jeffrey Dore went through the looking glass with Safari:
When using Safari [in Sierra], it suddenly enlarges the document, similar to what can be done with Control and mouse movement back and forth. If I revert to the previous screen, it returns to correct size, and I can click on the forward button, and it’s fine.
My suspicion is that Jeffrey has accidentally triggered a zoom option I just learned existed. With a trackpad or Magic Mouse, macOS offers an option to zoom on selected areas, just as you can in iOS.