RESOLUTION BUG

I'll translate what this image is telling you to see and this is how Photoshop should behave.

If you create a 10 by 10 inch document (we'll use this measurement for ease of calculation) with a resolution of 72 PPI and another document that is the same size but 300 PPI in Photoshop, you may notice that they are different sizes on the screen. This is due to the different number of pixels used in each inch. In the 72 PPI file, you can only fit 720 pixels across the document. At 300 PPI, 3000 pixels fit.

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You are not understanding friend that the problem is not the proportion of the two files being the same.

The problem is the resolution, image quality being the same in both files.

Easy to see in Photoshop.

Make a document that is 100x100 pixels @ 72.

Make another that is 100x100 pixels @ 300**

Set side by side and zoom both to 1600%.

Examine the size of the individual visible pixels: identical!

*set any value: no difference in size of the pixels.

Photoshop has behaved this way from day one, it is not alone. Not a bug.

The times PS “cares” about the resolution tag is when printing is concerned (the print dialog, view>print size, etc). 

All outlined in the PDF referred to in the 1st answer.

While it uses InDesign, the following video may help...

Image Resolution Explained in InDesign
https://youtu.be/BPUXXKobybQ

It makes perfect sense bro because that's how Photoshop has worked from day one (and not only PS) and because 1920x1080 at 72 and 300 and 3000 or any value is identical; 1920x1080 pixels. The ONLY difference is a metadata tag. This isn't a bug. 

A virgin file with 20ppi and one at 2000 at the SAME number of pixels HxW is, the same! 

Take a 1920x1080 image in Image Size and set the controls so resample is OFF (you are not adding or deleting pixels), alter the resolution value to anything you wish. It is the same (1920x1080). As the article provided (and there are many more), this is nothing new. It makes total sense if you concentrate on pixels, not metadata.  

It doesn't make any sense bro.

A virgin file with 20ppi should never display such a high quality image, just as it should never have the same weight as a 300ppi file.

Photoshop is crazy.

A 1080x1920 file with 20ppi should look like a LEGO, a Minecraft, only squares, it doesn't make sense what's going on bro.

Again, two documents, both the same pixels wxh but differing resolution tags. 

NOW open a third document and drag and drop over each, they produce the same results as expected. 

Sorry, I don't understand the language in the video but what I'm seeing on this end makes perfect sense. The two new documents have the same pixels but different tags. Dragging the 3rd document in either produces the same results; exactly what I'd expect. 

Here's a video link demonstrating the problem.

https://www.youtube.com/watch?v=gHJkdrYUcWQ

No friend, 1080x1920 pixels are just that, when I do as you suggest, they are identical. 

I don't know what you're really doing, maybe record a video.

I make two virgin documents, 1080x1920 pixels tagged at 300, one at 72, and drag one to the other, they are the same size; as they should be; they are both 1080x1920 pixels. 

Sorry, I've only been using Photoshop since 1990 <G>. 

Friend, I appreciate your help but I've been using Photoshop for 15 years.

If you drag the same image into a file with 20 resolution and into another with 300 resolution, that image will be displayed at different scales within each project/file, this is obvious, because each file has a different size, 1080x1920 is just the ratio of both.