QGraphicsBlurEffect not retina-aware on OS X

If I apply a QGraphicsBlurEffect to a widget as described at

http://stackoverflow.com/a/19386886/82195

the blurred contents appear magnified. It looks like QGraphicsBlurEffect isn't taking the widget's retina size into account somewhere. This happens for me here using Qt 5.7 and 5.6.1, but not 5.3.2.

Attached are screenshots showing a QLabel with QGraphicsBlurEffect applied and without. Also attached is a the Stack Overflow example I used to replicate the issue.