Tiny plot markers when using Retina Macbook and external Mac display

I have a Retina Macbook and Matlab R2013a works fine on it. If Matlab is already running and I plug my 27" thunderbolt display in, then my plots look fine. However, if I then restart Matlab with the display attached and make some new plots, all the plot symbols are too small. I can fix this on a case by case basis by making the markersize 12 instead of the default 6.

I'm sure this has something to do with a difference in pixel density between the laptop and external monitor. Any way to fix this?