Birds have a fast metabolism and a relatively simple gastric system which food moves quickly through.
If you have ever feed them purple or deep red fruits you see just how fast that comes out the other end.
Because most species evolved in abundant food availability. They are browsers/foragers eating all day. Not gut loading, like predators do. These small meals all day keep them flight ready , so can flee predators .
This is so different from us and our typical pets , that it's good to take a minute to think about and understand.
Most species also don't store fat very well. Exceptions some Australia species, budgie, tiel, some of the cockatoo. Amazon parrots can also be more prone to obesity. I've seen fat African greys as well.
Green cheek conure seem prone to being under weight for some reason.
I know my green cheeks eat a higher volume of food for body size than my quakers. Here at least my green cheek eats double the amount of food my quakers do!
Also metabolic needs change. When molting they can require double the base calories, at minimum 25% more their base calories.
Temperature drops of 5 degree fahrenheit can increase metabolic demand around 15-25%
So I always recommend having their pellets available