Well, it's a rather simple explanation. Cynthia is a female arctic fox, and Jack is a male European hare (and seems to be a tall guy, it's also written in the artbook). I did some research and I found out, unfortunately late, that in reality Jack could very well be taller and heavier than Cynthia. If you want, you can also take a look on Wikipedia or some other site that treats the biology of these two animals. Unfortunately, in the end it's Cynthia to be too tall to be an Arctic vixen, but patience.... actually there is a difference of a few centimeters between them (and Jack suffers for each of them).