Sure there is a need to comparatively decide which Rov is bigger. We need to in order to judge who is an authority in the first place. Otherwise you wouldn’t know who is a godol, in the first place. The fact that he is accepted as a godol only means that many people have judged his “level” to be that of a godol. And the same common sense that tells you so-and-so stands out among his peers making him an authority, tells you that certain so-and-so’s stand out even more. Or less.
Godol mimenu b’chochma ubaminyan is an assessment that it legitimately made.