To get the same daily gains, the forage animal has to move more dry matter through the digestive system period.

I presume a larger gut volume would aid in this.

But you could have a forage with a higher protein content (on a dry matter basis) than grain, thus making the digestion capabilities (or speed of digestion) of the two equal.

Where I see a real advantage of the larger gut is consuming forages with a high water content.