If it's still too big after command-zero, try command-minus a couple of times. That will shrink things with most common browsers.
It might also be useful if you identified your browser and its version, and the OS version. Different browsers might get different content from the server, or they might have different ideas of what the default sizes should be.
The problem occurs with Firefox V.84.0.2 Running on macOS 10.15.7.
I'm using Chrome right now and all looks good.
When I do the suggestions from @Weaselboy and @chown33 on Firefox the window is still screwed up.
It only happens on this forum and it started today. I don't believe I hit any setting accidently to get this, but I may be wrong.