I would say they're probably getting used to their new schedule and the new diet. I don't know how many diets you've been on yourself, but for me the first few weeks - especially the first few days - are the worst! Seems like you're always hungry, but then after awhile your body gets used to the reduced intake and you're not a starvin' marvin' anymore. Also, is it started to warm up there? A lot of time kitties will eat less in warmer weather.

If I were you I would just watch them and like Lacey said, if their behavior is normal, nothing to worry about. If they start acting sluggish, time for a trip to the white coats.