Don't the smaller dogs and more active dogs also have a higher metabolic rate than the larger ones or the less active breeds? It's my active dog that goes more often and the one that sleeps all day goes less often. They both eat all right and I'm not feeding discount dog food full of fillers, etc.

I know someone with a very small mixed-breed dog (weighed 3 pounds), and she commented to me that hers would drink a lot and have to go more often (both #1 and #2) than larger dogs she had owned in the past. If this holds true, I would expect the Jack Russell to go as often as he does, since the breed is both small and active.