These are primarily from stock that was imported in 1620's to Plymouth Colony, now Massachusetts, USA. (Some imports were also made in the 1800's) They were maintained as triple purpose animals. Around 1900 their production was equivalent to the then Jerseys and Guernseys in the US. Jersey production was then bred up for commercial dairying.
So, while they are from the same stock technically as Red and South Devons - things have changed in 400 years. This is why the Milking Devon is considered, by the American Livestock breeds Conservancy to be extinct in England. There are also Beef Devons, Ruby Red Devons and plain Devons - it's like Smith or Patel for cows
In the 1950's most of the Devon owners in the US began breeding for beef (and polled cattle) - American Milking Devons are descended from the stock of the stubborn breeders that maintained the triple purpose. The breed was almost lost, bicentennial celebrations of US independence caused historical groups to look for authentic early American livestock - and the breed was saved. There are currently about 600 registered cows.
Milk yield varies with genetics, but also with nutrition. High butterfat milkers give less volume. I keep my calves on, and only feed grass and hay, and haven't milked regularly. As an average expect only about 2 gallons per day of 5-6% butterfat milk.
The girls do look chunky in their pictures - they have their winter coats on.