Open source isn't entirely a labor of love, there are people that do get paid for their open source development. Unless you're ready to back up your claim with analysis of contribution quality based on employment status I could just as easily point to different properties of the open-source model as the driving factor.

No, I don't have statistical data, but I've experienced both sides in person.

The crucial difference is sensitivity to ROI.

When not paid, every hour spent programming is an hour not spent with your family, not spent attending to your hobbies, not spent resting or sleeping. Thus you do your best to keep everything as lean as possible.

When being paid, keeping things lean is still a nice idea, but you don't care that much any more. After all, you are going to spend same amount of hours on the project whether you keep it lean or not.