It has been noted that the Jersey cow began to be bred for a larger size and more milk production after World War Two when women began to leave home and join the work force and the family cow began to become extinct. I found an interesting article from 1905 that gives the typical production of Jerseys before they began to be bred up for production.
In part the article states:
"In two or three instances, however, the yields of the cattle are given, and it is interesting, in view of the performances of these latter days, to consider them for a moment or two.
Quayle gives 22 quarts (English measure) as the greatest quantity of milk given in 24 hours, the medium quantity being 10 quarts i.e. 5½ gallons, with an average of 2½ gallons. From April to August some extraordinary cows gave 14 lbs. of butter in the week, instances of 12 lbs being well attested. In summer 9 quarts of milk (English measure) produce 1 lb butter, in Winter 7 quarts. I presume the lbs are Jersey lbs.
Mr. Garrard gives 14 lbs. of butter per week as the yield of some cows, the yields of milk being from 3 to 4 gallons per day. "In one year the produce of a good cow in butter may be from 220 lbs to 230 lbs. (Jersey weight)."
Mr. Inglis had heard of a cow giving 22 quarts of milk, but according to him the general average produce was 10 quarts of milk per day and 7 lbs of butter per week."
http://jersey-dk.dk/index.php?page=jersey-home&id=906