Closest to perfect for pro tennis, height is perfect. About 6 one and a half. Build is perfect in terms of musculature development (not much and lean) but ideally would have a larger build/be wider. As he is though I'd say it's better to err on that side than being too big. Just big enough w the HT/lean weight combo that power is not an issue at all.
Two years ago there were a lot of players that looked like they trimmed down, both on WTA and ATP. Even Fed and Nadal looked slightly more svelt. But also, Kerber, Azerenka, Querry, Stan this year, etc. I don't think it is anything to do with Djo, but just the natural progress of finding the ideal athlete formula for players by trainers and sports medicine folks. Less weight fits both performance and injury prevention criteria.
To me, Djo is too skinny and may lack muscle that can help stabilize joints, but he makes up for it in flexiblity. Only time will tell though.