I know with dogs, when you switch them to a higher quality food, they don't eat as much. I would imagine the same could be true of any species. If it's better for them, they get the nutrients they need from less.
If you want to be extra careful, I echo the possibility of getting a scale and weighing your chin. Never hurts. I weigh the chins (breeders and rescues) every 15-ish days -- gives the quickest indication when something's wrong, as sometimes a slight change in eating (not super noticeable, but there), over a stretched out period of time, can amount to a big difference in weight. If the weight's steady or the chin's gaining, then you know everything's fine.