I think the proper reference would point to bulding capacity (as in population density). Not thicker concrete.
Now you know the Bronx like I do, and population density means nothing from the outside looking in. You know on average on Walton, Girard, Davidson Avenues and even Grand Concourse there can technically(by design) be one building chopped up into 6 buildings, just on 1 block. And you also know that not EVERY apartment in that building is occupied. And hasnt been for maybe 20 years. You can see buildings that face one street follow through all the way to the opposing side of the block and then look up and see bricks where windows used to be. Entire wings of buildings sealed off(awaiting gentrification). If anyone disputes me I ll take a picture next time I m in that area. What could have in the past(~1950-60s Bx, pre-CBX) housed 200 apartments and probably 10000 residents now may have the same # of residents(higher pop. density) but lower occupancy rate.