sniper, you need to triangulate your physical abilities (talents and strengths) versus your competitors. its a principle from economic Game Theory.
for Fed and Nole, to add a few kmph on their serve, it might even be a negative economy, because they have their power on their serve, their placement, plus they have adequate power on their groundstrokes.
however, what they seek, is that 210kmph ace down the T, in the 5th set, after 4 hours. They dont need to be the Canuck Serb Milos Raonic and hit 230 thunderbolts out wide, and 235 down the T, in the first set. or match del Potros groundies. They can do the the rope a dope, and look to get the big guys over the long run in a 5 set match.
It is best them to get their defense to the best on Tour, and especially, peaking for the Slams like Nadal. The Slams are won by defense.
It is no surprise, that the best 4, are the best defenders on the Tour. And they can switch from defense to attack in one stroke, and not a hail mary, it is a proper stroke, and not a get-out stroke to kill the rally which is probably a lottery chance. The best defend, and never give up, but can find openings even from the defense.
Now this is where the guys like Fed and Nole have used their drug regimens to focus on, for the, cough cough, marginal gains. Their gains by puting on 18inch biceps would be negative economy.
Now Serena figures, the 18 biceps, she can just destroy her opponents and out hit them, and their defense and speed, and their opposition arsenal of power, wont be sufficient. If Serena lost 10kg and became a speed demon, she would be less the player. If Nole or Fed put on those 10kg, then Nole and Fed would be less the player, because their strengths and weaknesses only become manifest when they play their competition. Anna Ivanovic lost about ~10lbs about 5 years ago. "lbs, not kgs, I have switched from imperial to metric back to imperial".