Which could thus indicate a mature individual, built for speed.
Unless being fleet footed is not much of an option with the larger individuals, so bulk would have been the way to go, as any elephant can attest - although thta would incude the build of their young.
It's been proposed that perhaps T Rexes hunted as packs, with the younger doing the running down and the adults taking care of the rest, but that makes sense for a whole 5 minutes given their cranial capacity and something that big better off solo.
Also could be that the younger ones not only had to run after prey, but run away from larger predators, including their own species if they were cannabalistic.
But nano was also found in earlier strata if I got that right, and makes sense they lineage was smaller at some point.