Bear in mind what is and what isn't significant amount of potential nutriants in the context of the discussion.I picked those numbers to sort the people who let their birds free in an urban or suburban "back yard" and those whose birds can range in a space that (in most environments) is likely to provide a significant amount of potential nutrition most of the year. 1,000 sq ft per bird is equal to ground worthy of 1/2 AU per acre which many soils can achieve with only minimal human effort.
One would have to set anything over 10% of their total intake of foodstuffs as significant in this case.
One would also need more detail of what forage is available than straight square footage. For example, a half acre backyard with a working compost heap would give a higher nutritional rating than a few flower beds.
One other factor that often gets overlooked is how chickens feed and forage.
It's not easy.