Feathers get damaged over time, whether from the rigors of flight, infection or exposure to UV light. The bigger the feathers, the longer it takes to replace them. Eventually, birds have to spend too much time replacing feathers and not enough time feeding or finding mates, thus there is a limit to how big a bird can get.
Most small birds regrow all of their roughly 20 primary feathers once a year. Birds that molt this way have a limit of about 3 kilograms. Any bigger and this process gets too time consuming.
Birds larger than this will usually do one of two things. Either they will stretch this process out over two or three years, or they will regrow all of their feathers at once and forgo flight for that time. A third, rarer, process of replacing multiple feathers at once is used by some birds as well.
All of these are trade-offs. If you replace all of your feathers at once, you can’t easily feed or escape predators. If you replace more than one at a time, it affects aerodynamics and could make things harder for the bird.
Even with these different strategies, there is a limit on size. The largest flying bird ever discovered was the now-extinct Argentavis magnificens which lived around 6 million years ago. This bird weighted around 70 kilograms. The largest soaring birds today weight around 20 kilograms.
