Originally posted by romeck A FF sensor size is ~2,4 times larger than APS-C one.Correct?
It depends.
Canon APS-C implies a conversion factor of ~2.56 and even when you take a single manufacturer like Pentax where the conversion factor is often close to ~2.25, the actual factor changes with each sensor. I'm not sure whether all of the 23.5x15.6 area quoted for the K-3 is optically active. Usually, there are at least a border of dark pixels to be subtracted, etc.
To simplify matters, the crop factor for Pentax, Nikon, etc (but not Canon) is typically assumed to be 1.5 even though it may be closer to 1.53 in reality. The square of the crop factor is the area conversion factor, hence 2.25 being commonly used.
@Ogl: Your pixel density example happens to yield the correct result, but only because the sensors have the same pixel density and their areas differ by the APS-C <-> FF area conversion factor. Normally, working out the area conversion factor using the number of MP would not work at all.