My parent's cocker spaniel is the same way. I've never seen a dog eat so little yet be so active! He's been a poor eater since he was a little puppy, and they've switched foods for him several times over the past year and a half. They've tried wetting the food, adding incentives like yogurt, veggies, even cereal, and he just doesn't like to eat! He is supposed to get about two cups a day, but usually ends up eating only about 1 cup. He is just under thirty pounds, and very solid - no fat on him, he's all muscle! Just goes to show that every dog is different, and those 'feeding recommendations' on the food bags are just generalities!
I wouldn't worry too much about Finny. Is he energetic? growing at a proper rate? is his coat healthy? If you answered yes to those, then Finny is probably fine - he's just watching his figure! As always, though, talk to your vet if you are concerned!