It turns out that zip codes aren't as populous as you might imagine.
As of the Y2K census, only a dozen zip codes have over 100,000 residents -- 80% of all zip codes have less than 15,000 residents, with the median zip code at 2,500.
Some zip codes have zero residents -- a few large office buildings have their own zip codes, but nobody actually has a home address there. Examples: the Empire State Building, the Pentagon, the former World Trade Center, IRS tax processing centers.
It turns out that zip codes aren't as populous as you might imagine.
As of the Y2K census, only a dozen zip codes have over 100,000 residents -- 80% of all zip codes have less than 15,000 residents, with the median zip code at 2,500.
(Source: http://www.census.gov/geo/www/gazetteer/places2k.html )