A report by an American social statistician, Lewis S. C. Smythe, is the most reliable source for estimating the population of Nanjing at that time.
It estimates the population just before the Japanese invasion at up to 250,000. How could the Japanese army have killed 300,000?
300,000 Chinese were killed by Japanese soldiers in Nanjing in December of 1937. These two Japanese soldiers had a killing match, one soldier killed 105 Chinese, and the other 106!!!