On OS X the Packed Pixels display won't automatically show up as 'Retina' or 'HiDPI' resolution, instead the OS will use the full resolution of the display making everything tiny. This can be changed using the 3rd party app (non-free) SwitchResX.
First, download SwitchResX.
Switchresx Custom Resolution
You can play with the resolutions for the Packed Pixels which is probably listed as Color LCD (2). However not all of the HiDPI resolutions work due to an OS X bug. If you are happy with any of the ones that do work then stop here, else continue.
SwitchResX can automate resolutions switches for all applications Assign a Display Set to an application, and automate changing settings when the application starts, comes to front, or quits. SwitchResX can save your desktop icons and windows, and replace them of course. The third option, as first spotted by MacRumors forum reader 'plazmic', is the sophisticated SwitchResX shareware application that offers extensive control over 'Monitor Resolution, Color Depth, Video Mirroring, Display Rotation, Display Overscan' and more. It even allows one to create and enable new resolutions.
Switchresx Macos
If you're on El Capitan or later you need to disable System Integrity Protection (temporarily) to add a custom resolution for Packed Pixels. Follow the instructions from SwitchResX creator.
Now add a Custom Resolution that is 'scaled'. It must be 2 pixels bigger or smaller than the native resolution in one direction (this is the OS X quirk). I found bigger makes it slightly blurry so go with smaller—I chose '2046x1536'. Hit cmd+s or File => Save. Then reboot.
Once rebooted start SwitchResX again and choose the resolution you created in the Current Resolutions tab. Note it will be halved! So in my case it's 1023x768.
Switchresx App
Switchresx M1
If all looks good you can turn on System Integrity Protection again (recommended). For me at least this procedure resulted in a beautiful clear picture on the Packed Pixels, just what I'd hoped.