puppy isnt getting any bigger

Question
Hello, I'm so glad I found this site! I have a 5 month old male lab and he only weighs 25 pounds. He has been to several vets and they all say the same thing, that none of his bones show, so he isn't too skinny, he's just very little. The rest of the litter that he was in go to the same vet we do, and his litter mates are between 40-50 pounds right now. He's been checked for parasites, and the vet can't find a reason why he isnt growing. I recently switched him to California Natural puppy and he has gained a few pounds in the past week, he is looking a little bigger. Before CN, he was on hill's I/D (vet's orders, since he was having diarrhea and loose stools). While he was on hill's he didn't gain a single pound. My question is, do you have any ideas on why he's not gaining weight? Could it just be the food? (he was on hill's for 6 weeks). Is it likely that he's just "small" even though his litter mates are almost twice his size? Sidenote:he's very healthy otherwise. Thank you SOOO much! sorry about the long question.

Answer
Not all puppies become the same size, start out the same size, grow at the same rate. I would probably think that the Science Diet probably didn't help.  

A dog at that age should be eating 3-4 cups a day.  If the vet says he's healthy, just keep feeding him the good quality food and see what happens.  My guess is that he'll catch up.

In my last litter, the smallest girl is now the biggest girl.