I'm having similar issues with my Macbook Pro 13" retina display.  The start buttons look about half the size that they are supposed to be.  I'm assuming it has something to do with the fact that this is a 2560x1600 display and it must be scaling it for some UI elements, and not so much for others.  The button in the screenshot is not a default button, but one that was downloaded from WinCustomize.  

Here is a screenshot of that:

As requested, this is how it looks on my late 2013 Macbook Pro 15". The DPI settings are at default, set to "Larger", i.e. all the way.

Please note that while those arrows might seem ok on your desktop, because the retuna DPI is expanded, but on Macbook 15" retina they look real tiny, especially the ones in the menu against items. Left+Right padding are tiny on all arrows.