There's a couple things.
Males tend to show more VERTICAL growth than their female counterparts.
Light influences growht rates.
The amount of N in the diet does too.
And strain is also important. Some ar pretty darn frisky. My OGKush has grown as much as a couple inches a DAY, and my Yeag's MTF put on an inch PER SHOOT overnight. I have to cut clones tonight or I'll be in trouble.
Oh, and F1 hybrids are known to be fast growers- a phenomenon called 'hybrid vigor' ... look it up!