Oh, both are so cute. The young baby should be close to same size as the 6 month old. I have 3 cockatiel babies that recently turned a year old. The last one hatched 2 and 3 days after 1st 2. I noticed, before they grew all their feathers, that his crop was not as big as that of the other 2, and he was 1/2 the size of them. I put him and "daddy" bird in an extra cage with seeds, water, corn, pellets and millet spray. I closed the cage door, and mom and other 2 babies were in their usual cage, with door closed. We had to go somewhere, got home about 4 hours later. Our little runt's crop was nice and full of food, and I put them back in their regular cage. In a day or 2, he grew to size of the other 2. He would "line up" with his siblings for food, but they were not giving him a chance to get fed by the parents as much. But, after that I guess he figured it out how to get in there by parents to get enough food. And, he has turned out to be very vocal, and was first to fledge. But it did take him 2 more weeks to wean completely than his siblings, but he was eating food by himself, but still wanted to be fed by parents longer. - It could be he may not have been getting enough food or they weaned him too early, possibly to get him to the pet store with the other one, which is terrible Both of them have feathers that look a bit scruffy. I too would want to buy them, to get them out of there. I don't know with him being about 5 months old how much more he will grow. Mine were all the size of their parents before that age, but if you buy him offer him millet spray, pellets, peas, corn along with seeds, good quality. Bulk food is cheaper and fresher if they sell it in your area. And if there is one in your area, take him to a vet. At any rate, even if he doesn't grow much more he will hopefully be healthier and happier. One idea too is maybe talk to the owner or manager of the store and ask him if he would consider a discount if you buy both birds, pointing out how little that one is compared to the other one. I hope things work out for you and the birds.