Actually, I even find the fights not as long... usually they take three chapters at best (with exceptions of Luffy's fights, who are against the 'baddest guy in the arc'... those go above that number).
Compared to "Naruto" (which, to be honest, I'm slowly starting to get bored with) that's not so much. For example in the Rescue mission arc, each fight lastest like one volume, which makes around 9 chapters. Each fight and there were basically six (with Naruto vs. Sasuke taking up even more space).
Or take even "Bleach"... nothing can beat the length of those fights. I think Kubotite is aiming to make them DBZ-long and although nothing happens between the end and beginning of the chapter, fights has been taking several chapters lately.
OP case is different: I find the fights exciting and although they always involve the same characters, they're not as repetitive like those in "Bleach" ("Naruto" also sometimes repeats tricks, but at the very least Kishimoto-sama tries to make those battles interesting by making the fighters think ahead).
Oda-sama simply has great imagination and as long as the battles will keep on being original and action-packed, I won't mind them getting few chapters. So far there has only been one fight that I think lasted too long in the entire series: Luffy vs. Eneru/Enel...