Calzaghe is far too big. He used to walk around at 200lbs without looking chubby. SRR on the otherhand used to make 147 and fight on the same day a few hours later for 15 rounds, so he wasn't even struggling (and fight weekly or monthly or fortnightly so was always 147 year-round!).
A natural Supermiddleweight against a natural Welterweight? And it's ridiculous to say that the Supermiddle would UD the Welter???