I have a set-up where I have a retina MBP (bottom) plugged in to a non-retina display (top).

Normally Matplotlib looks fine on retina or non-retina displays when used separately, but in the case of mixed resolutions, the fonts and the elements of the figure appear too large on the non-retina display. The following image sequence shows a matplotlib window that was created on the retina display and is then moved to the non-retina display. The same issue happens if I create the figure in the non-retina display to start with, so this is not about moving from one to the other.

I then click in the bottom right to resize the window: