If it fills my screen, then it is full size, neither zoomed in nor out.
When I look at 720p content on this same screen, at screen-filling size, it looks nowhere near as pixelated as the pic in question.
Stop being stubborn man.
A 720p monitor showing a 720p picture will be natural size when using full screen.
If you use the same 720p monitor to show a 4k picture it will crush everything to downsample so you will lose to much detail that you may not see a diference against that same other 720p image, but if you allow you to show a 1:1 (or as you insist in call zoom in) then you'll see a lot of difference.
But if you prefer to look at a better comparison, pick up a 4k monitor and look both pictures, the 4k image will fill the whole screen, the 720p will only cover 1/9 of the screen (then yes if you ask it to do full screen you'll have a zoom in) and it will look terrible, but you wouldn't game using 1/9 of the screen so that how a 720p image shows on a 4k screen.
And of course you haven't really seem a big leap in 4k screens in store, most of the time they aren't even showing 4k content.
You are basically doing confirmation bias, putting yourself in situations that you can't see a difference and saying the ones that you can see the difference are wrong, when it is actually the opposite. The ones you see the difference are the right way to compare.
duduspace11 "Well, since we are estimating costs, Pokemon Red/Blue did cost Nintendo about $50m to make back in 1996"
Mr Puggsly: "Hehe, I said good profit. You said big profit. Frankly, not losing money is what I meant by good. Don't get hung up on semantics"
Azzanation: "PS5 wouldn't sold out at launch without scalpers."