If post kt birds evolved the dinosaur body plan many times more or less, why they never evolved the gigantic dinosaurian sizes? They had much time to do this before the true ascendance of the large mammals, and they have better adaptations for this than mammals, like pneumatization of the bones. Yet there are modern mammals that surpass even the largest living or extinct bird by many tons.

One factor is that birds are bipedal -- and due to the evolution of their forelimbs into wings, they can't easily revert to quadrupedality. That restricts their size, which is why there are no sauropod-sized birds.

Another issue is that birds don't have heavy, counterbalancing tails as big theropods do. Birds tend to be front-heavy for that reason, which has to be compensated for by holding the femur near-horizontal to shift the centre of mass forward. That means the femur is under a bending load, which is more stressful than simple compression. That is part of why there are no tyrannosaur-sized birds.

More generally, specialisations for flight all tend to reduce weight, and are hard to undo.