Makes sense now that I think of the fact that Atlanta is known as the "city in a forest" and is said to have more tree canopy cover than other US cities.Atlanta as well as other southeast cities are mostly ultra low density suburbs and leapfrog development. Even their cores are basically like Richmond Hill or Central Etobicoke in terms of density.
The median for Singapore is about 3,900 people per subzone, with a median area of about 1.23 km^2. That said, the population in these subzones varies much more than the US census tracts -- some subzones have as many as 70-120,000 people, whereas many others have a population of zero (like the Central Water Catchment subzone which is some 65 km^2). However, within each neighborhood density doesn't vary as much as in the Americas -- many of these subzones are essentially a cluster of high-rises, with the park next to those high-rises being a subzone of it's own.What's the average (or median) land area and population for each of the units of measurement for those cities? Their size can have a fairly big impact on the final values. For census tracts, they're usually about 4000 people each. However, I think for Toronto if you used dissemination areas (roughly equivalent to US block groups) of about 600 people each vs the larger ~20,000 person "neighbourhoods" from the map in my previous post, there would be an almost 2-fold density difference.