While there are always exceptions to the rules a pure Jack Russell will typically only weigh between 15 and 25 pounds. Is it possible that you are over feeding him at all? I have an 11 year old Jack Russell who weighs in at 16 pounds which is about average for her size.
... Even bark at thunder. Q. How big do Jack Russell Terriers get? A. Jack Russell Terrier's grow to be about 12 - 14" at the withers and ...
Are you sure that he isn't overweight? What does your vet say? He could just be bigger than the average Jack Russell Terrier.
It shouldn't get much bigger than that. I know because my friend Liz has one and hers is about that size.
Yes he will not get bigger than 40 pounds.
I just had to put my Jack down of 13 years. He was always bigger than the "normal" jack size. Not fat, not overfed, actually in very good shape; just bigger. I've been looking for that some dog but they are all typical, normal, smaller jacks. I want the bigger version.