Actually, it's easier to make smaller screens. It's not like the iPhone needs to have the same number of dots as the mini squeezed into a 4" space -- that would be insane.
For instance, the iPhone 5 is 1136x640, and iPad 3/4 is 2048x1536. See how the retina iPad has a much larger number of dots than the iPhone? But when we look at dpi, the iPhone is 326 dpi, while the iPad has 264 dpi. That is, the total number of dots isn't directly related to pixel density.
Anyway, what you'd need to do to make a "retina" iPad mini would be, for instance, to make a screen double the dimensions of the iPhone, so, 2272x1280. And this is the same dpi as the iPhone 5, so you could make this screen with the same process as doing it for the iPhone. However, you'd need to make sheets with a surface area 4 times that of the iPhone, and with every additional square inch, the chances of getting a defect increases. So trying to make larger screens is harder.
Again, you aren't trying to squeeze 2048x1536 dots into 4 inches, you are trying to take a 4 inch screen with 1136x640 dots (326 ppi), and make a 8 inch screen (well, it won't be 8, it'd be 7.xx, but I don't feel like doing trigonometry to calculate the exact length) with the same 326 dpi.