Shannon was even bigger in the latter stage of his career. He was leaner when he younger. I'm sure Briggs weighed about 20 stone when he won the world title against Sierhei Liakhovic, but he was still in great shape.
He was huge 260lb's+ He was physically impressive, but not athletically he couldnt really box 12 rounds, he relied on strength and power
While he is a great deal slower (even prime), less mobile, easier to hit, and lacks a single weapon as potent as Tommy's hook, Shannon's chin was orders of magnitude better. He could probably risk standing up to a few of Morrison's best shots, whereas it could only take a few ponderous clubbing blows from him to spark Tommy out.
Yeah his asthma played a part in that and i remember him saying before that he doesn't prepare himself for 12 rounds, he just prepares himself to knock the other guy out
Morrison was smaller, but quicker and more athletic, the Morrison that beat Foreman, knocks out Briggs in The late rounds...
the jab would play a big part for Briggs and keep Morrison off balance and at range, then brings the right hand behind it. this lasts no more than five due to Morrison's chin. Briggs has this.