I think in length/size they're about full size when they wean.
Might be time to find a new vet. Birds might gain some muscle with age but they are essentially full grown by the time they wean.
But I'll use Mera as an example. She weaned by herself with me at 5.5 months old. She was only 800-850grams at weaning. By a year old she was 1000grams and by 1.5years old she was 1150grams.
So they do keep gaining weight after they wean. Most babys at weaning age weigh less than an adult would.