Although I don't have the figures, I suspect that by the time you factor in all the infrastructure for a unit (elevators, common areas, hallways, parking (?), etc.) you don't really cut out all that much cost by reducing units by a few hundred square feet.
Seaport rents are particularly extreme for reasons that I confess I don't fully understand but you definitely get into diminishing returns as you decrease apartment size.
$2,000/mo for 420sqft