First, he's not fat. He may have more body fat than the average ATP'er, but he'd still be a stone lock for top 1% of fittest people on the planet.
Second, do you wunderkinds know what fat IS? It's stored energy. The reasons fat has a bad name are, of course, many. Yes, there's a strong correlation between body fat and lack of fitness -- but by no means an absolute correlation. And yeah, fat is in many senses "dead weight," meaning you must have extra reserve strength to move it around at the same speed, or for the same duration.
Now, that said, look at the guy's freaking legs. They're made of granite. He obviously HAS more strength down there than the average ATP'er. So he's having no trouble moving a little extra weight around.
Is he in good cardio/endurance shape? Obviously. The guy, if anything, is FAMOUS for routinely getting into longer-than-expected matches -- and wins more than his fair share of them.
So, he's got the strength to move the weight around well enough to be an athletic threat against any player. Furthermore, he's got the wind to stay tough for 5 sets, over and over again. So with those things in hand, what does body fat do for him? It helps him. Over the course of a match, nobody is going to burn so much fuel so fast that fat reserves begin to deplete for energy. But over the course of two weeks? You can bet they will. At the end of two weeks of gruelling 4 and 5 setters, all else being equal, would you rather have the body of Nalbandian or Monfils?
The answer ought to be Nalbandian. But of course, it isn't for most. Because in the modern society, looking good is more important. When Fed shows up with a little belly, or Nalbandian shows up a little stocky, they get #### for it. Which is absurd. Monfils is built to get from point A to point B faster. Nalbandian is built to get from point A to point B "fast enough", fifty-thousand times over the course of two weeks. Monfils has a better body than Nalbandian for track & Field. Nalbandian has a better body for tennis.
Combine his physique with his patience and his ability to hit textbook groundies all day, and Nalbandian has the perfect build for his style of tennis. There's a reason he's been a top-flight player for years without the raw talent or weapons most of the top guys possess.