It would help if you specified an exact resolution or set a scaling factor to force either Kali or the virtualization to double (or more) the pixels on a Retina display.
In VBox, try the following;
1- Download and install Oracle VM VirtualBox Extension Pack from their official site here.
2- Open virtual box, go to preferences, go to display, then change the scale factor to either 150%, 200% or 300%, depending on your preferences.
3- Restart the running virtual machines.