Make sure monitor is in native resolution. On most PC’s you can right click on the desktop to get to display settings and on the Mac you can go to Apple Icon (upper left) — About This Mac — More Info. This will display your screen resolution in pixels (image 1).
Is your Photoshop opening all images at 300ppi? This is a setting you can change in PS. Whatever the imbedded info with your image PS.
Will open it at whatever you have set it to, or if you have not changed from the default at 72ppi. A newspaper (no names) of my acquaintance would angrily call me up because the 6x4 images I was emailing them were not the 300 ppi that they demanded. The picture editor(joke) did not know of the default settings. He blamed - for some time, I add - 'his incompetent:D photographers'.
This may, of course, have nothing to do with your problem but it was worth telling (I think) Regards.
Open Terminal app and run this command: systemprofiler SPDisplaysDataType. You will see output that looks like this: $ systemprofiler SPDisplaysDataType Graphics/Displays: Intel Iris Graphics 6100: Chipset Model: Intel Iris Graphics 6100 Type: GPU Bus: Built-In VRAM (Dynamic, Max): 1536 MB Vendor: Intel (0x8086) Device ID: 0x162b Revision ID: 0x0009 Displays: Color LCD: Display Type: Retina LCD Resolution: 2560 x 1600 Retina Retina: Yes Pixel Depth: 32-Bit Color (ARGB8888) Main Display: Yes Mirror: Off Online: Yes Built-In: Yes.