Things being equal is the key. Usually, they are not. To Wit:
Up til two weeks ago, I rode my Trek 830 MB on our club rides of up to 45 miles. It has the old Ovaltec (I love it) chainrings and only 18 speeds, steel North Road bar, heavy sprung saddle, rack with day pack, Ford-size fenders, and me at 230 lbs and I'm 50 years old. It now has 1.5" Armadillos which I do feel make it roll alot easier than the fatties. The set-up is all of 44lbs.
On hills, I smoke 'em. They have to be very strong to outlast me to the top. I'm breathing down their necks coming down, too. Upright and all.
How? Well, like I said, things ain't equal. I believe the Ovaltec (see Sheldon Brown's BioPace article under his glossary B) gives a real advantage going up hill. The guys pushing round rings just poop out, that's all. Even though their bikes weight 10 lbs less than mine. BTW, BioPace is extremely effective at near stall speeds as well, allowing very precise power application in tight situations.
Down hill? You can't beat round chainrings, that's a fact. But, this is where my weight comes in handy. Gravity favors the Clydesdales, just like a tractor/trailer rig.