Answer
An apartment building has less area in its outside walls exposed to the weather than multiple single-family units totaling the same volume.
For example, eight 1-unit cubes have a surface area of 48 square units, but a 2x2x2 apartment building only has 24 square units.
The smaller area means less heat loss per unit.
Another way to think about it is that in an apartment building, more of the rooms share a wall or floor or ceiling with another warm room than with the cold outdoors.
This is discussed on page 236, Figure 12.16.
