When it comes to evaluating their heights on the shoulders, the biggest Jack Russell is only three inches shorter than the Rat Terrier. However, the difference between their weight is more noticeable, with the largest Rat Terrier weighing eighteen pounds more than the identical Jack Russell. Rat Terriers weren’t bred to keep up a certain form and size for a sure operate. As a outcome, they are often one of many larger sizes so far as small breeds go. The Jack Russell, however, was bred to be compact and fast, which makes them slightly bit smaller and lighter. There are many differences between the Rat Terrier and the Jack Russell.