I just re-watched part of Ali v Frazier 1 and noticed how much bigger Ali`s shoulders were than they had been in his perime and how small Frazier looked, Joe looked faster than Ali, was this because he was lighter and would this have given him an advantage over modern heavies too?
Generally, yes. Joe was at his peak fitness for his bout, reflected in his weight. This is also combined with the fact Ali was heavier, which may have helped his sturdiness in taking Fraziers shots, but reduce his speed.
Yes he took Frazier`s hook better than Coopers due to his extra weight, but do you know how that works?