Golden retriever puppy?

hi there, i have a golden retriever puppy at home. she is 13 weeks old. she is below the average weight. she only weighs about 8lbs(4kg). i am feeding her with eukanuba(lamb). according to my vet, her coat isnt that smooth as well. he recommended me to go for royal canin. please help me out as i am kinda hurt that she is really tiny. i am feeding her thrice a day as well.