(5-20)  Our localisation energy expression is:    Ek =  3h2/32mπ2<r>2

Since the energy is proportional to 1/m, it makes sense qualitatively that the nucleus should be more localised than the electron, as the attendant energy cost is less. But it’s not that simple: the proton mass is about 2000 times that of an electron which means that <r>2 could be 2000 times smaller – and therefore <r> could be about 40 times smaller – for the same localisation energy.

However the difference in scale is much bigger than this. This means that although the bigger mass helps to reduce the energy cost of the extreme localisation of the proton, the biggest factor must be a much stronger attractive force than the electrostatic attraction that localises the electron. The strong interaction is extremely short ranged but at distances in the region of 10-15m, which is around the radius of a proton, it is indeed much stronger than the electrostatic force.