Gliders and sailplanes have wings made for maximum lift and, hence, relatively lumbering flight. Fighter aircraft have wings made for aerial maneuver and fast flight, but nowhere near as much lift. For that reason, if a C5-A is the largest transport plane which modern technology allows us to build, and it is, then you do not expect to see a fighter plane five times the size of a C5-A, the reasons being fairly obvious.
Likewise, the albatross and certain condors are bird equivalents of sailplanes and transports and, at 30 - 35 lbs. or so, albatrassos have enough trouble with takeoffs and landings for sailors to call them "gooney birds". You therefore do not expect to see an eagle or a hawk, or anything like that, basically a bird equivalent of a fighter aircraft made for maneuver and pursuit, five times the size of the largest albatross, for the same reason which you do not see a fighter five times the size of the C5-A.
If, on the other hand, you DID see something like that in prehistoric times, then you have to figure something was very different in the extent to which gravity itself, the limiting factor here, operated in prehistoric times.
The original article on the Argentinian teratorn from Bioscience, December 1980:
Argentine scientists' have unearthed the fossil remains of what Seems to be the world's largest known flying bird, Argen- tavis magnificens. The bird lived be- tween eight and five million years ago, in the late Miocene era. With a wingspan of 25 feet, the bird measured' 11 feet from beak to tail, and weighed in at l6 to 170 Ibs. Its first wing bone, the humerus, was approximately 22 inches long.
Paleontologists Kenneth E. Campbell and Eduardo P. Tonni identified the fos- sil remains at Argentina's La Plata Mu- seum. Working with leg, wing, and skull bones, Campbell and Tonni, have tenta- tively concluded that the enormous bird probably did more soaring than flapping. They admit that it seems initially unlike- ly that a bird of that size could even get off the ground, but believe that the size of the wing bones and their markings in- dicate that Argenravis magnificens did fly. "It has the right size wing bones, and it has the markings on the wing bones of secondaries, a type of flight feathers," Campbell said. "It's unlikely that a bird would have feathers and wing bones suit- able for flight if it didn't fly."
In the past, there have been larger birds and larger flying animals, but no larger flying birds. Pterosaurs, giant fly- ing reptiles, were the biggest creatures to take off; one pterosaur found in Texas had a wingspan of 30 to 33 feet. The larg- est previously known flying birds were the North American Teratornis incred- ibilis and a marine bird calied Osteo dontoris orri. Both had wingspans of al- most 16 feet.
Based on his studies of another tera- torn fossil, Teratornis merriami, Camp bell believes that the teratorns were predators. "The long, narrow hooked beak and the type of jaw mechanism found in this species are similar to those that would be expected of a bird that grabbed small animals with its beak and swallowed them whole," he said.
Note the reference to the same dilemma which I've cited all along:
They admit that it seems initially unlike- ly that a bird of that size could even get off the ground, but believe that the size of the wing bones and their markings in- dicate that Argenravis magnificens did fly. "It has the right size wing bones, and it has the markings on the wing bones of secondaries, a type of flight feathers," Campbell said. "It's unlikely that a bird would have feathers and wing bones suit- able for flight if it didn't fly."