As you guys mentioned Chicago, I decided to put its numbers together:
Downtown Chicago
---------------------- 2020 ------ 2010 ------ 2000 ------ 1990
Near North Side --- 105,481 ---- 80,484 ---- 72,811 ---- 62,842 ----- 31.1% ----- 10.5% ----- 15.9%
Loop ----------------- 42,298 ---- 29,283 ---- 16,388 ---- 11,954 ----- 44.4% ----- 78.7% ----- 37.1%
Near North Side has 6.8 km² for a density of 15,500 inh./km² and Loop 2.9 km² and 10,800 inh./km². 20 and 6 census tracts, respectively.
Loop, from a CBD, turned into a very dense residential area. Ditto for the southern tracts of Near North Side, which are the ones growing like crazy. The other areas are more stable as they've always been residential.