My understanding of the theory is that animals which subsist on greens have a huge gut to allow for fermentation to get the calories from the fiber in the plant

My understanding of the theory is that animals which subsist on greens have a huge gut to allow for fermentation to get the calories from the fiber in the plants. Eating meat allowed our brains to grow larger and gut to shrink, since meat is more calorie dense, and the brain consumes a lot of energy.