The discrepancy is clear in the two photos displayed here, which show the National Mall shortly before Obama and Trump were sworn-in around noon eight years apart.
No doubt Obama had a larger crowd but the Washington Post is not a source to rely on.
Strangely enough if you download the referenced photo (at a higher res. from the NPS - https://www.nps.gov/aboutus/foia/foia-frd.htm) used by the Washington Post in their article the clock tower shows it was taken at 1:15pm, over 1 hour after Trumps inauguration had finished.
— 11:47 a.m.: Presidential oath is administered by Chief Justice John Roberts
— 11:51 a.m.: President Donald Trump delivers inaugural address
_ 13:08 pm Trump, Pence and their wives attend a Joint Congressional Inaugural Committee luncheon
