
Different complete feeds can affect the amount of weight gained. I have found on Chudleys, Eukanuba or Gilpa Valumix my dogs pile on weight. On any Skinners, Omega Working dog or Gilpa with green muscle extract they stay about right. I have found that a very mixed diet works well with my dogs. They get fresh meat and fish with biscuit, and a complete food every day. They are fed twice a day, as this suits my breed. I know other breeds owners feed only once a day as they say this is best for their dogs. If I have a seriously under weight dog I usually feed a boiled breast of lamb fat and all. I was told this is excellent years ago by a vet, for putting on weight on animals who do not hold weight easily or are under weight due to illness or neglect.
I agree that your dog's breeder maybe more knowledgeable about the breed and his/her own breeding lines, therefore they would be more helpful to you. Another thing to take into account is your bitches age. In some breeds they are "skinny" at this teenage stage of developement.
Hope this helps.