The official documentation is at https://www.mathworks.com/help/matlab/creating_guis/dpi-aware-behavior-in-matlab.html
My suspicion is that when you set the experiment to appear on the second display, that it is looking and seeing that the second display does not have Retina density, and so decides to stop effectively magnifying objects on the laptop display. An object that is (say) sized as 144 pixels would normally display on a Retina display as 144 pixels / (72 nominal pixels per inch) = 2 inches physical. But if somehow that scaling got turned off then you might start to display as 144 physical pixels. Your Mac Laptop is probably roughly 226 pixels per inch ( exact value depends on model and year see https://en.wikipedia.org/wiki/Retina_display ), so the object would start displaying as roughly 144/226 = 0.64 inches.
I do not know how to change or turn off this behaviour. I notice in the description https://www.mathworks.com/help/matlab/ref/matlab.ui.root-properties.html#buc8_0n-MonitorPositions that if you unplug or plug in new monitors or they otherwise change sizes, that you need to restart MATLAB to update.
I also see in https://www.mathworks.com/matlabcentral/answers/1767375-error-in-multi-screen that Psychtoolbox warns about multi-display. In the particular messages in that Question, which was for Windows, the messages say that you will get strange results if the second monitors are not the same resolution as the primary monitor. The messages there say you should
help RetinaDisplay
for more information.
