It isn't just the size of apartments, it is everything about modern building codes. How far the setback from the sidewalk is, how much green space is required,

It isn't just the size of apartments, it is everything about modern building codes. How far the setback from the sidewalk is, how much green space is required, parking requirements, all contribute to a lack of density. And certain types of housing, e.g. boarding houses, aren't even legal anymore (moral crusaders shut them down long ago!)