T. rex stuck to small prey?

As I've said, all Tyrannosaur news is automatically considered cool. Even this article, which suggests the biggest theropods were not actually good at bringing down large prey. Two scientists argue that the paucity of fossils of young herbivores, compared to the large number of eggs that we know were laid, indicates predators were mowing down the youngsters. This and other evidence implies that even the biggest predators avoided the stupendous battles with adult Triceratops, Ankylosaurus, etc. in favor of small stuff they could gobble whole.

COMMENT: It all makes sense - to a point. Predators will chomp anything they can find - the more defenseless, the better, although with an appetite like that of an adult T. rex, there is some minimum size beyond which prey wouldn't be worth the effort.
Here are my (admittedly amateur) thoughts, though. My reaction to this theory is the same one I had to the arguments T. rex was primarily a scavenger. Either way, the damn thing is just over-equipped. Nature may make mistakes in the evolutionary process, but she doesn't turn out animals with greater size and more armament than needed - that would be wasteful. A predator six or seven meters tall, with a head the size of a Smartcar and serrated teeth as big as bananas, just doesn't seem logical unless the demands of getting a meal - the results of the constant "arms race" between predator and prey - required it to become that big and that well-equipped. The carnage among younger herbivores might well have been due to the smaller predators, like Velociraptor, as well as juveniles among Tyrannosaurus, Gigantosaurus, etc. What makes the most sense to me is still the idea that T. rex specialized in big prey.