Hi! A raw diet does consist of a good amount of protein, however, it also consists of bone and offal (organ meat). There are different schools of thought on raw diet. Some keep to purely protein, organ, and bone while others also include some carbohydrates such as potatoes, and also other vegies. Typically in the wild, wolves did not eat much carbs or veggies. The carbs, veggies, and grains that they ingested were most often thru ingesting the intestines and stomachs of their prey.
Feeding a raw diet requires much research and committment. It is not an endeveor that one can or should go into haphazardly. I have been feeding raw for about 2.5 years. Raw food items are digested and absorbed more quickly by the body. The dog's body also utilizes more of the food it eats and as a result; the dog tends to eliminate less often than when feed on a kibble diet that has more fillers to it.
A raw diet is not for everyone. I am a supporter as I have seen some wonderful benefits in my own dogs from it. But it takes research to determine exactly how you want to pursue it. I think your question for large breeds is a valid one. I do not currently own a Mastiff but maybe someone else will be along that can answer that for you. I think you are wise to consider the specific needs of your breed and your dog. It may be that for an extra large breed that you may want to utilize other food items as mentioned above. However, when looking back at wolves.... Wolves are pretty large animals and survived on essentially raw food/protein with the exception of probably the items in the intestines and stomach of their prey as well as the occassional grass.
Good question. I hope someone else can contribute to answering you.