Totally different bodies and styles. Hearns was more top heavy so his legs weakened in the late rounds after carrying all that top weight. When he gained weight he had even more trouble holding all that weight. Tommy had much better boxing instincts that he still managed to be great.
it is not about skinny legs or weak, it is about who had the skinniest legs in proportion to their upper body. And that was Hearns. But he had the most power also. Roger was very top heavy also.