Firstly, you should see the foxes that live near my parents house in East London/Essex! There about the size of a large retriever/labrador!
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Secondly, look at the size of birds! They originated from dinosaurs! But evolution made them smaller as it helped with their need to fly!
Look at us Humans, there is quite a big difference in the accepted "normal" hight of a man now and a man 100 years ago, also compare our hight to that of a Gorilla, we are much taller, but not as broad!
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Its all down to evolution and natural selection!
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A firther example is Great Danes and Jack Russels, both dogs, both from the same genetics yet a HUGE difference in size!

the same reason why chiwawas and newfoundlands originated from wolves domestication and/or evolution