I bet Europe and North America historically offered much better free range fair. Average size of the free range roosters in the study was around 2.5lbs. Historically free range gamefowl in Europe and North America were a pound or two larger than that at least.
It only stands to reason that diversified small farms with other livestock present provided a richer environment than large monoculture farms, suburban backyards, or even the sort of rural area I live in -- which consists of 3-5 acre lots, mostly wooded^ with some lawn and some scrubland post-logging, and no livestock other than an occasional flock of chickens.
Also you'd get a richer environment when you don't have large-scale use of pesticides.
^NC Sandhills -- very poor, acid soil growing primarily Longleaf and Loblolly pines with some scrub oak in earlier succession and sweet gums near water sources.