Volume and, since most physiological items are about the density of water, weight or mass of poop likely depend on two factors. First is how much you drink during and shortly after exercise. When I head out for a century, I often forget to drink enough; but when I'm on tour, after a day or two I'm pouring water down my gullet and though a lot comes out as sweat, stool size recovers.
The second factor is fiber in your diet. Increasing your intake of granola, fruit, or adding a half dozen prunes daily, coupled with adequate fluid, should increase your stool volume within a few days.