Maybe it's the smaller acreage and the diversity (of animals and crops). In my 13 acres I have goats, sheep, horses, ducks, chickens, pigs so 5/13= 0.38 species per acre. Can't be bothered to work it out but there is probably a version of this formula that creates a pivot point from smallholding to farm at the right level.
Alternatively, a more philosophical answer is based on why it matters. If it does not matter if it is a smallholding or a farm then there is no difference. Legally I imagine it is all about how involved the EU want to get chasing compliance on tiny units rather than focussing environmental and food chain enforcement on bigger units. Lifestyle wise it is more about reliance on market forces, commercial vs lifestyle choices, scale Vs diversity.