He looks fine to me. I stole this from wikipedia:
"There are currently no size classification s for the Goldendoodle. It is difficult for a hybrid litter to "breed true"; that is, it is difficult to know exactly what size a Goldendoodle will grow to as an adult, regardless of parental size. However, Goldendoodle sizes can vary if they are standard or mini. A standard Goldendoodle is the result of crossbreeding with a standard-sized Poodle. A standard male Goldendoodle can range in height from 21 to 24 inches (54 to 62 cm), and weigh 55 to 85 pounds (25 to 39 kg). A standard female Goldendoodle can range in height from 20 to 22.5 inches (51 to 58 cm), and weigh 45 to 65 pounds (21 to 29 kg). Goldendoodles can also result from a female golden retriever and a male miniature or toy poodle (both via artificial insemination). Such Goldendoodles can range in height from 13 to 21 inches (33 to 53 cm), and weigh 25 to 45 pounds (11 to 20.5 kg). Goldendoodles also have different coat types. They can range from curly, wavy, or straight depending on its genes. Goldendoodles can have different colors, depending on the poodle genes passed onto the pup. However, Goldendoodles can be white, blonde, tan, cafe, chocolate, red, black, silver, parti, phantom, or a mix. A mix of colors can come from the same litter."
Butter looks to have the more slender shape of a poodle than the robust physique of a golden. He may just have a naturally high metabolism.
If you are sure it's the kibble causing his monthly episode of diarrhea and not treats he may be getting like yogurt/cheese/pigs ears/bones to chew on etc etc I would change.
I feed Canidae Chicken and Rice to 2 of my pups. I don't really go by the feeding guidelines on the back. It lists puppies to 6 months then jumps to adults. Giant/large breeds are still puppies way past 6 months so I don't feed the adult portions until much later. So don't stress about that extra cup you're giving him. Try giving him an extra cup on top of that if you feel he is too ribby. I would split up the kibble to a breakfast and a dinner though.