Yes, but only in the short term. Researchers found that dogs fed a high-protein diet grew faster and heavier than dogs fed a low-protein diet. But after 4 weeks, the dogs fed the high-protein diet had grown no heavier than those on the low-protein diet. They also found no effect on size at all when the two groups were fed the same diet for 8 weeks. So in the short term, it matters, but after four weeks, it doesn't.