=) Here's more low calorie healthy items that I freeze in kongs... besides those already mentioned:
plain cooked chicken
raw pieces of chicken, beef, turkey, etc.
pureed veggies
mushed bananas (with a little bit of peanut butter mixed in)
green beans
cottage cheese
In my experience, calories do make an impact, but the BIGGEST & BEST way for a dog to lose weight is simply activity. Especially in breeds that have been bred to work a lot. Frequent walks, trips to the park, play sessions with other dogs, and just moving around more, is sure to get a dog to lose weight. My BC used to be about 5 lbs overweight, not noticeably but enough to annoy me, for a couple years! I tried low calorie foods, feeding way less, no cookies, all of that... with no results. And, even though he played fetch & frizbee for over an hour every day, after getting another dog and him being active more often through out the day, he is in perfect shape and actually lost 10 lbs (so he's now very much on the lean side, but as healthy as he could be). And I am not skimpy with his food at all anymore!