It is the exact same size as buttons on the iPhone if you are rendering the iPad 2 design on the iPhone as well, but in that case the button on the iPhone would be less than 40 points wide and also too small by Apple's guidelines. I am saying that it would be against Apple's guidelines on both the iPhone and the iPad 2.

That's why I mentioned an iPhone-specific design, as well. The iPhone's pixel density isn't a problem because it can be distinguished from the iPad 2. And, as you've shown, if the iPhone-specific design is displayed on the iPad Mini then there is no problem at all. The issue is that there is no way to distinguish between the iPad 2 and the iPad Mini.