Precipitation, growth, and coarsening of Si3N4 crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680oC has been studied. Contrary to the common observation in kinetics, coarsening rates of crystals in length and width are found to accelerate when the total volume of crystals remains little changed. This is attributed to the concomitant β-Si3N4 to β′-SiAlON transformation, which introduces an additional driving force for crystal dissolution and reprecipitation. As a result of the additional driving force, which has a nonmonotonic size dependence, the normalized size distribution is expected to evolve with time, initially broadening, then shifting skewing as the transformation passes the midpoint, and finally converging to a sharp distribution as the transformation completes. These evolutions have been observed in all the compositions studied.