As a general rule, when your working on a screen, forget about physical dimensions and resolution (PPI). All that affects the size you see something on screen is its pixel dimensions. Your screen doesn’t know what an inch is, all it knows is pixels.
In your examples:
- 1×1 inch image at 72PPI is 72×72 pixels.
- 1×1 inch image at 27PPI is 27×27 pixels.
Your screen shows those images at their pixel size, ignoring any physical dimensions or resolution settings.
If you were to print those images they would both print at 1 inch square. The only time the resolution really comes in to play is when you print.
If you were to import your images in to say InDesign, they would both show at 1×1 inch because InDesign is resolution aware. This is because InDesign is made for (or at least was traditionally) print design—where resolution really matters.