I’ve encountered this bug for many years now and it shits me to tears.
OS X has this great little thumbnail resize bar at the bottom right of Finder’s status bar that lets you change the size of photos you are viewing in Finder. It’s a great way to explore your travel photos, without any 3rd party software. However, inexplicably about half the time it seems to just not work.
Well it is a glorius day as I found the solution. You can fix this by toggling the status bar off & on. The easiest way to do this is via the shortcut (twice) ⌘/
Thanks to Tony T1 for the tip.