I am not much of a dog person, since I've never owned a dog myself.
I asked around for you, and got a few answers I am not sure if they are helpful.
Simply copied and pasted them
Answer #1.
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I have yet to find a clear guideline for canine nutritional needs and this makes it very difficult to come to a decision on dogfood... the guidelines for humans are not even right. I know of two cases of dogs in record books, both ACDs, that lived past age 25, one was on a strictly vegetarian diet and the other was on a raw diet.... so, lol, that doesn't help any! Yes, I do suspect that vets are pushing brands because they are being sponsored by them, much like human doctors prescribing medications from the parmaceutical companies that sponsor them. It just comes down to a lot of research and ruling out any allergies that your dog might have... and making your best judgement and hoping you make the right one. I know many have success with a grain free diet but also that many foods labeled "grain free" just substitute the grain with simple starches... and excessive starches are said to be even worse for the health of a dog. I also think it is a wise choice to not just depend on the food itself for everything a dog needs. Appropriate table scraps and supplements and a daily multivitamin are all great measures to ensure that your pet is getting a broad variety and surplus of nutrients into their body.
Just like with humans, it is important to read the ingredients list and avoid foods that contain man-made chemical dyes or artificial chemicals... those things should not go into any living body.
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Answer #2.
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Dogs' size is not always governed by the size of their parents. (My mom was 5'10" & my dad was 6'2". I'm 5'4"!) It is a good indicator but not an absolute. Your vet is correct- she is probably just going to be a smaller dog.
A large breed dog like a Golden should absolutely be on adult food at that age. The food you feed will not change the size she will be BUT it will help determine how healthy she will be.
The cost of the food is not a good indicator of the quality. Many pricey brands such as Science Diet, Eukanuba & others are junk. Unfortunately, Royal Canin is not one of the better foods. You can do much better for the money.
READ the labels & compare! That's the best way. If you don't know what the ingredient is, it should not be in the food. Corn is not dog food. They do not digest it well.
Most of the foods labeled as Holistic are reasonably good. Chicken Soup is a great food for the price.
http://www.dogfoodanalysis.com/
http://www.dogaware.com/
If you choose to switch your dog's food, do it very slowly so as not to cause digestive upset. It should take at least 10 days to make the complete switch.
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Answer #3.
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do NOT feed Beneful. It's utter crap. Don't feed anything with wheat, soy, by-products and corn. Beneful is loaded with the last two, which are also fillers, your dog would get no nutritional value
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