This theory works in the fact they used to have a separate species called "Nanotyrannus" which turned out to be a juvenile T-Rex, and it has been determined that, at a younger age, Tyrannosaurus was an agile predator that hunted its prey in packs, but as it grew larger and older, it used its size to go after larger, slower meals that could fill it up, where it had to conserve its energy instead.