Thinking about it, many tyrannosaur species were huge, but they lived alongside even more gigantic herbivores. So a pack of pack of tyrannosaurs may have been comparable in size to their prey as a pack of wolves attacking a moose. The prey has to be small enough for the predator(s) to bring down, but big enough to feed all of them. A pack can reliably bring down bigger prey than a solitary predator.
Also I'm totally up for writing this if anyone wants to ask for it during a relevant prompt call. :D