The difference between adult dog food is the contents. Kibble size will obviously be smaller in puppy food for ease of chewing and minimized choking hazards. However, what you should be most aware of when providing your adult dog puppy food is the higher percentage of protein, calcium, fat, and the increased number of calories. However, be sure to check the percentages of each brand you are considering as each can vary vastly. Especially in calcium contents which can be a health hazard for adult dogs.