I don't think it'll go down to size. Kalambay was Benitez's stature, although heavier but Benitez was pretty solid at 154. Don't forget that he was only 21-22 years of age when he stepped up to light middleweight.
I don't know what McCallum's true height and reach are, I've seen them listed differently in various sources. The tale of the tape for Kalambay-McCallum I lists McCallum at 5'11, 74 inch reach, Kalambay at 5'9, 72 inch reach.
Kalambay was quicker than Benitez imo, he also used the ring differently than Benitez imo, Benitez used tight little turns and stayed closer to his man.
McCallum also avenged that loss to Kalambay.
Benitez head movement would not be as effective either, not against a man renowned for hitting the body to find the head.