The others are correct. I remember taking a UA DC-10 flight IAD-SFO in 1981. The airport was very quiet. The plane had many empty seats.

Around 1987, I flew through IAD on TWA to CDG. The airport was still quiet. TWA used the mobile lounges.

By 1992, the UA midfield terminal was somewhat busy. In the 10 years since then, IAD seems as busy as other large airports.