I never said that the older players had an advantage in that way, its more that they no longer have much of a disadvantage physically, due to the way sports science works these days.
Since they are usually better than the younger guys technically and tactically, they tend to win out.
At the very best, they can achieve equal results in cardiovascular scores, such as those I've mentioned. However, in overall relevant elements of fitness, they should be at a disadvantage in agility, flexibility and potentially strength and power due to loss in flexibility and the possibility of injury. So overall, there is no advantage at all to being an older tennis player, physically speaking.
Regarding Murray, that's generally what happens when one gains muscle mass and loses fat. I've seen it happen 1000 times.
He must've went from a weedy 75kg to around 82kg. It's what we call "noob gains" in the bodybuilding circle. This along with the addition of scruffy facial hair can't hurt.