How is it possible that Couture lost to 205 Liddell, but he beat 270 Sylvia. How can a fight be more setup?

Sometimes i think they will throw a fight but who knows. Its strange to me that he lost to Liddel whos half the size of Sylvia, but beat the snot out of Sylvia lastnight. Strange to me also but I love the UFC. But it was a kick fight and Im glad Couture won.