An acre is not a farm.
An example in North America - "With only 1½ acres cultivated in permanent beds, the farm grosses more than 100 000$ per acre with operating margins of about 60 per cent, enough to financially sustain his family." http://www.themarketgardener.com/
"an internationally recognized 10-acre micro-farm"
Even 10 acres is considered "micro". Also, that's 1.5 acres of cultivated permanent beds on 10 acres.
For a 1 acre farm, where are the homestead, barns, equipment(tractors, tillers, etc), animals, etc located? All of that takes up space, which can't be used for growing. You need more than a single acre to actually have an acre of growing area.