Maybe he was faster because muscle doesn't necessarily slow you down... if the mass he gained was fast twitch fiber then why wouldn't he be faster?

Also on paper it sounds like Roy gained a lot more weight than he actually did...

he may have weighed in on 79kg when he fought as a light heavy but he was probably 85kg+ by the time he got into the ring and would have walked around at 90-95kg outside of training camp.

For heavyweight he will have gained some mass, but not as much as the difference between a light heavy weigh in and a heavyweight weigh in would make you think.

heavy weights don't dry up for weigh ins...