I'd like to add, when I switched my cats to a raw-food diet (why do we feed grains to carnivores anyway?) I noticed a side benefit that their stools were much less frequent and voluminous. In addition to their increased well-being, it's one of the best benefits, at least from a litter-scooping perspective.

Anyway, I read that, for cats at least, it's because before when they were eating grain-based dry food they were excreting most of it, while now they are using most of what they eat. So I'd expect the same is true for you. Taking out the carbs also removes a lot of bulk. Plus aren't the by-products of protien and/or fat metabolism passed more in the urine?

Good luck on the constipation remedies and don't worry about the decreased "output"!