I used your own formula, i.e. when we double the number of pixels, the resolution gets multiplied by the square root of 2. So the factor is sqrt(2)/2.But, you also got the calculation wrong. The 21 mpx sensor has 6.4 µ pixels, the 50.6 has 4.14 and so the expected increase in spatial resolution is 6.4/4.14 = 1.54. That is not much more than the 1.42 measured.