Why do JRT became suitable for hunting badgers and others?

It's just something that puzzled me. I ask a friend what breed of dog was best for hunting rats and badgers and he told me JRT(Jack Russell Terrier) is the best. Then he added some brief history. What puzzled me is what he said about that JRTs are also good for hunting FOXES. So I was thinking, Are they that strong and too aggressive knowing that foxes are almost the same in terms of body built, like height, etc.?