How large can flying birds get in Earth gravity?

The largest flying animals of all time were pterosaurs like Quetzalcoatlus and Hatzegopteryx, which could weigh up to 500 lbs and have wingspans of nearly 40 feet. The largest flying birds, such as Pelagornis and Argentavis, had wingspans of around 25 feet and (in Argentavis's case) topped out at around 175 lbs.

This is because flying birds are limited in their size by two factors. One is that birds rely only on their hind-limbs to take off. On the ground, their wings are dead weight, and this is especially true the bigger they get. Therefore, once birds reach a certain size, they tend to become flightless. Pterosaurs used all four of their limbs to take off, much as bats do today, and therefore had no reason to lose their wings. This is probably why no flightless pterosaurs are known, and why there aren't any flightless bats today. The other factor is that birds have feathers. The larger a feather is, the longer it takes to grow in after it is molted. In raptors, this is mitigated somewhat by the fact that they undergo a sequential molt, but in truly gigantic birds, losing even one feather would compromise their ability to fly.

So with all of that in mind, how large can flying birds potentially grow? Is something like Argentavis the hard upper limit? Or is it possible for different conditions-- say, an environment where the benefits of retaining flight at large sizes outweigh the costs-- to allow for something larger?