Patricia Clark:

Urbanists often lament that developers no longer erect the small apartment buildings that were once a staple of city neighborhoods. Instead, they construct single family homes or large apartment buildings.
 There are good reasons to revive this “missing middle,” however: Small buildings are a good way to add density without compromising the character of quiet, single-family districts. They also provide a convenient way for older homeowners to downsize without leaving their neighborhoods.
 But the best reason is that smaller apartment buildings are often cheaper for renters.