The Newtonian gravitational constant G is one of the most important fundamental constants of nature, but still remains resistant to the standard model of physics and disconnected from quantum theory. During the past >100 years, hundreds of values of G have been measured to be ranging from 6.66 to 6.7559×10-11 m3kg-1s-2 using macroscopic masses. More recently, however, a G value ((6.04±0.06)×10-11 m3kg-1s-2) measured using millimetre-sized masses shows significant deviation (by ~9%) from the reference G value, which the authors explained is resulted from ‘the known systematic uncertainties’. However, based on the observation of historical G values and the protocol of the millimetre-sized masses based experiment, we proposed a theory that this deviation is not from ‘systematic uncertainties’ but actually G will rapidly decrease when masses sphere diameter is less than 0.02 meters. More important, the proposed theory matches the measured data very well, suggesting that G may indeed decrease rapidly when mass diameter smaller than millimetres.