The District has the fourth smallest average home in the United States, according to a report by PropertyShark. Boston has the smallest homes; Orlando the biggest.
In fact, DC is one of four cities that have smaller home sizes than they did a century ago. The other three cities are New York City, Miami, and San Francisco. The new home sizes in DC are shrinking because more apartments are being built than houses to meet the need of soaring city population, according to the report.
District of Columbia Median Home Size from 1910 to 2016

In the rest of the US, however, the average size of homes has increased consistently over the past 70 years. Now the gap between the District and the rest of the country is becoming larger. Only for two decades in the last century did DC exceed the average number, from 1920 to 1939. DC’s average home size hit its lowest point in comparison with the rest of the US in the latest time period PropertyShark measured—1,241 square feet less than the national average.
Comparison between District of Columbia House Size and US Average Size

If you are curious about other cities, here is the full report.