I can't find the original study on this; from what I'd read, they only checked that longer than average is correlated with women who report higher vaginal orgasms. I agree it seems odd to leave girth out of the equation.
Of the many types of orgasms, this study focused on just one (one which typically requires rubbing around the cervix more) so yeah it's not surprising longer = more orgasms there. I kind of love feeling an orgasm from going deep. It's always a little surprising when it happens (I'm generally thinking more of myself than about getting her off) but feels great.
It's just one of many kinds of orgasms, and I'm sure for the shorter-dicks out there, they can get in closer and grind the clit more easily, and average guys are nicely positioned on the g-spot. I don't agree that this study means bigger = better, just that everyone's better at different things.