Tubby here weighed in at 39 lbs at the vet's today!
We feed him measured proportions of kibble and he gets regular exercise.
The big culprit is, of course, his consumption of human food via Norah. She drops a lot of food, hands him a lot of food, and Scooter will out-right steal a lot of food from her -- he will literally snatch things right out of her hands if she's not vigilant. The vet said that cutting back on his regular kibble or switching to diet kibble won't make much of a difference; he has to eat less (preferrably, no) human food. So, I guess when Norah's snacking or having a meal, Scooter is gonna have to get shut away in the bedroom.
That's my best thought -- any other suggestions? Maybe doggie lipo?