I get what you're asking. When I put my dog on a raw diet, his poops shrunk to almost nothing because EVERYTHING that went into his mouth was being utilized by his body, and less was being wasted to come out as...well, waste.
But their ideal and optimum diet is totally different from ours. Canids are 100% carnivores, while humans are omnivores. While we can absorb all the nutrients from fruits and veggies, there's gonna be stuff left over. Think like the pulp tray of a juicer after all the juice is squeezed out.