Murray's fitness and physique has been spoken about as if it's at some mythical level for too long imo. He is a great athlete but the fact they have gone out of their way to tell us how tough he is when, out of the top four, it's him who looks the most haggard and starts getting sloppy with his movement/execution the quickest is a little amusing. The amount of times I've seen him blunder points because he was run ragged compared to Federer, Nadal or Djokovic (2.0) is tenfold more.
That said, each body has its traits which aid aspect of physical ability but may hinder another - if even slightly. I personally think he's on the big side for a modern tennis player in the legs. This will be of benefit for some things but certainly can have its down side too.