Genome size evolution is a fundamental problem in molecular evolution. Statistical analysis of genome sizes brings new insight into the evolution of genome size. Although the variation of genome sizes is complicated, it is indicated that the genome size evolution can be explained more clearly at taxon level than at species level. I find that the genome size distribution for species in a taxon fits log-normal distribution. And I find a relationship between the phylogeny of life and the statistical features of genome size distributions among taxa. I observed different statistical features of genome size distributions between animal taxa and plant taxa. A log-normal stochastic process model is developed to simulate the genome size evolution. The simulation results on the log-normal distributions of genome sizes and their statistical features agree with the observations.