Cometary split phenomena have information on the structure of cometary nucleus. Although the absolute value of the size or mass of each fragments of Periodic Comet Shoemaker-Levy 9 could not be determined, the relative ratio of the brightness can be roughly estimated. Assuming the homogeneity of all the fragments, we derived the mass distribution index as 2.3 +/- 0.1, which is larger than usual values of asteroids, periodic comets, and fragments of collision experiments. Some of other examples of split nuclei of Sun-grazing comet group show similar properties. This may suggest that the original nucleus was made by an aggregate of planetesimals of similar size. In this paper, we compare these derived size distribution with theoretical expectations from some formation theories of planetesimals.