I can't give you the technical reasons, but yes, it's easier to produce power if you have room to open up your chest and a larger angle at the hip. Sitting aero reduces drag but also reduces power output, so you have to find a happy medium. I would guess that big, strong riders (rel. large leg muscles) would prefer a more upright position than small, lightweight riders, since the difference in drag/frontal area isn't as big as the difference in power output. I go aero at the drop of a hat and really can't produce much power on the flats compared to my bigger riding buddies.