Larger abdominal muscles will make your waistline larger!
Really though, if someone goes from sedentary to less sedentary while maintaining the same food regimine, sure they'll lose some weight, though they'll quickly stall. Maybe this is where you are right now?
Barbell back squats and overhead presses have really done a great job making my abdominals stronger. Something I've just started messing around with is the ab wheel, that thing is brutal too! They're easily made or cheaply purchased. Also there are lots of bodyweight abdominal exercises out there too. Good luck!