The iPad mini is thinner than the iPhone 5. Its form factor and the weight are the selling points for this device. The iPad mini has a weaker processor because it doesn’t need as much power; the screen has the same resolution as the iPad 2. While the iPad mini definitely is a lot more than a shrunken iPad 2, it is still the best comparison there is when talking about the hardware.
So the iPad mini is really thin, and it doesn’t weigh much. These are selling points, and they hold true.
What will happen when it gets a retina screen? This isn’t a device that can be much heavier, it would be against its purpose, but it might have to be if it is to get a retina screen anytime soon.