Is that possible without calibration? A small finger pushing hard would register the same area as a larger finger pressing soft. I can't imagine it working like that.
what you are saying make sense if it only ever sees the contact of the small finger while pressing hard. but if it first detects the small finger only pushing lightly, and then after pressing harder it causes more finger to come in contact... making the size larger... it would know that it is a smaller finger pressing harder. it's not just one moment in time, but multiple.