Just a thought, and maybe not relevant at all, but...
Maybe your cat was producing more poop before because of the higher carb diet; because there was more of that food that was indigestible and being passed out as waste?
With my own cats I found that, as I've reduced the carb content of the food (and have fed better quality food with less fillers etc), the poop seems to get smaller (and less smelly too...). And my feral girl who eats mostly raw food has the least poop of all (and her poop doesn't smell!). It seems to me that the more digestible the food, the less poop comes out the other end...
As I say, just a thought....
Another thought though if it is constipation is to increase the water in his diet, either by adding a little water to the food, or by getting a water fountain to encourage him to drink more..?