Let me see if I can explain this well enough and hopefully someone can answer very simply. I just got a new laptop, reinstalled Photoshop (so whatever the latest version is), and when I sized an image to be 540px wide/400px height the actual display in my Photoshop was much smaller than it used to be? I'm a pretty simple Photoshop user, I just use it to gif so I really don't know the ins and outs of PPIs and all that jazz. I do know that now that this is happening, my GIF quality is much lower once saved than what it looks like in Photoshop (on the too tiny supposed 100% zoom). I'm probably doing a sorry job explaining and I apologize for my ignorance but I just wanna make the HQ gifs I once made with my cheaper, slower, old laptop and I'm pretty sure this wonky scaling (is it considering "scaling") issue might be to blame.
You seem to be misinterpreting what 100% zoom means.
100% zoom is not a physical size. It means 1 image pixel mapped onto 1 screen pixel. So the physical size will vary on different screens according to the pixel density.
Some browsers scale up images when showing them on high pixel density screens (effectively using 4 screen pixels to show 1 image pixel). Photoshop does not, and must not, do that as it would impact the critical adjustment of images. If you do want to emulate what those browers do, use 200% zoom.