As i feel this a simple thory that was mplemeted time ago and now it has come to a better level. When i serched on web for the technology it expalined here.

This is how you see the screen if you look at the screen without glasses.

This Keypad has 17 keys in a area of size 9.5X6.5. So If we calculate the area of a unit key, that will be 3.63 (cubic centimeters per key). But the good thing is this keypad is extremely easy to use. I can type super fast even without looking at the keys.
Then a general calculator. Normal size of a general calculator will be either same as that of a Keypad or less. We'll amuse that it is of size 8X5. So the area of a unit key will be 1.53.
And this is the most probable answer for my question. It's super cool. I know there are plenty of keys inside this tiny area. BUT who are you going to press those?. The area is around 2X2 cm in size. And have 16 keys on it. I don't see any use of all the keys if you don't have a pin. So the unit area for a key is 0.25.
The Other keypad that everybody users now a days is the mobile keypad. This is taken from the N95. it has 12 keys in a area of 4.5x2.5. So the area of a one key can be calculated as 0.93. This key pad is far better sample of a compact design.
And if you count the number of keys there that is 13. And the area taken by a single key is 0.46.