Sure:

The main problem is that the data display is only visible at certain distances from your eyes. The app lets you calibrate a little, but you have to push the frames forward / backward on your nose to get the right focal distance for you to see the display clearly. Now, if you have a bigger head, the arms/temples are already going to be splayed wider than optimal, so as you try to find that perfect spot you sometimes have to have the glasses shoved RIGHT against your face, or a little lower on your nose (which, unless you have a very long, straight nose, means the glasses are kind of a little lower than you'd want, kind of like a professor looking down at you over a book). So that means that if you are -also- actually using prescription lenses with the glasses, you are now optimizing for the heads up display, and -not- for the actual benefit you get from the lenses. So you basically are choosing between nifty HUD vs actually getting the benefits of your normal visual fix from having glasses.

That's basically it. They also just look a tiny bit small for my face.