being 6 foot 6 helps. del potro has enormous easy power off the ground with that size.
he has also improved his serve so he is very difficult to deal with. poor guy is still without a single masters series shield.
that is just how tough it is today.
as for nadal, he is clearly spent. he has used up quite a bit of mental and physical capital in taking all that he has taken. winning 10 titles with such a demanding style of play would kill just about anybody else. he may have hit the wall.
this is why I cant see nadal taking anymore titles this year.
I don't necessarily think so because if you look back in history there are no champions who were taller than 6'3" that won multiple Slams. In fact, 6'1" is the ideal height or should be. No point in having so power if you can't execute it but more importantly, the movement is essentially compromised. None of Del Potro or Isner or Raonic are known as good or quick movers - far from it in fact. What helps them against a player like Nadal is the surface of the court. If it's a bit faster then these players can hit through him using their power. Also, they are all more or less big servers. A couple of them, namely DP and Isner did see some success against Nadal on clay as well but that's a bit exception to the rule. In the end, they didn't get the results and Nadal was still the winner. These players also need to be relatively fresh against Nadal so that's another element. Otherwise they are nowhere as deadly.
Anyway, Nadal was sweating like crazy against Del Potro while DP looked completely dry. I don't think the weather was as humid; perhaps Nadal sweats a lot more when he's under pressure or feeling a bit nervous? I don't know.