Originally Posted by LeeD
Federer is 6'1" and about 175 lbs.
If that's your definition of scrawny, so be it.
By professional athlete standards he is VERY scrawny. Even by tennis player standards he is scrawnier than most.
To make some anecdotal refs (like so many do here, which of course usually means nothing)...
....when I was 24 playing hoops about 6 hours per week and doing no working out other than a few dumbell presses and push ups, at 6'1, I was about 185 with body fat probably around 12-13% (probably the same range as Fed judging by how he looks with his shirt off) depending on how many milk shakes I had that week. Not showing a six pack, but no love handles and couldn't pinch an inch.
Fed, scrawny, maybe exaggerating, skinny, definitely.
Not to get too far off the point, the simple fact is you don't have to be very big or very strong to hit a tennis ball hard. Your time is better spent on learning how to hit the ball more effectively through technique than trying to find some aspect of your physique that will help you hit it better.
If someone ask you: what should I do to get a great forehand....build my biceps up like Nadal, or learn technique like Nadal or Fed..the answer is obvious.