I disagree here. I think he has gotten a lot stronger but not just by brute strenght. Crocodile brought Luffy two time near to death (three times if you count the poison) and Luffy was coming back. Luffy just came back because he grew above himself. And no matter how much you train, such near death situations are experiences that you will never get through training. Luffy´s power is based on determination and with each fight he surpassed his limits in that. That´s why I think that he has gotten a lot stronger after he defeated Crocodile. Not in terms of pure strenght. But that alone doesn´t determine the total package of fighting skills.
However I think that he has gotten a lot stronger in Skypiea. Mostly because of the environment. As a snowboarder I can assure you that if you go out for a little jogging in a hight between 1200 and 2100 meters above sea level for a week, you will definitely be much fitter when you get back home and do your regular workout then. I´m telling the truth.
And now the last component to that difficult environment. Luffy was carrying around that huge golden ball with him. And he did that above a hight of 10.000 meters. He even climbed the Giant Jack with it in order to get to Enel. And then in the end he was swinging that giant ball around to stop that Raigoh. That is one hell of an workout.
Just remember back at his first confrontation with Wiper. Wiper was fitter because for him the thin air was normal environment. Just imagine how much fitter Wiper would be if he would have the chance to breath the air of the Blue Sea. This whole situation is actually similar to the whole DBZ concept where they are training with high gravity.
For me Luffy definitely has gotten physically stronger in Skypiea. [qimg]http://www.wackyb.com/zzub/style_emoticons/default/flex.gif[/qimg]
I totally agree with all of this Ivotas. Luffy has gotten stronger, not only in strength but his experience and will through life and death situations. In a few words Luffy gains More Stamina and Endurance in fights. His ability to endure is very key and the reason why he is able to overcome the odds.
Remember the race is not given to the swift or the strong, but to the one who Endures.! Words of wisdom for life.