The test is use, not acreage
A large lot with a garden is still a residence. Land that produces income, carries livestock or stores commercial equipment is an operation, and a homeowners policy is written for the first and excludes much of the second. Owners frequently sit on the line without realizing there is one.
A farm policy covers what a homeowners form will not
Barns, equipment, stored crop, livestock and farm liability are all contemplated by a farm form and largely absent from a homeowners one. If those things exist on the property, insuring it as a residence leaves the most valuable part uncovered.
Farm liability is a different exposure
Workers, visitors, equipment on a public road and animals that get out are farm liability rather than personal liability. A homeowners policy was not priced for any of it, and a claim arising from the operation can fall outside it entirely.