As far as I know, American ferrets are much smaller and less muscular, but it could just be due to the fact they are sterilized at around 4-6 weeks of age and fed crappy kibble. They are also more prone to adrenal disease and insulinoma compared to their European and wild counterparts. European lines are often crossed with polecats to make them a more robust working animal.
But in general, the general gist of domestication is to retain the juvi mental and physical traits.
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