My source for the estimate is on DC's metro council.
And it gets 400k+ every year, so I'm not sure why you're not believing it this year.
Someone on the DC metro (do you mean city council, or do you mean someone on the transit system's metro board?) council doesn't know how to estimate crowd sizes better then anyone else. I am sure they have claimed 400,000 for years, but it doesn't mean those estimates were correct either.
I looked into it and found lots of people before the event predicting 500,000 based on who knows what, but neither the park service nor DC police estimates crowd size.
Lets look at this logically, let say each person took up a space 3X3. Which frankly if you marching is not that much space as that is about a foot in front and a foot in back of you, and 6 inches on each side. Try walking with someone a foot in front of you, it is really hard.
That gives us 9 sq feet per person. If there is a half million people, that gives us 4.5 million square feet for all of the marchers. I'll be generous and say constitution avenue is 100 feet wide (based on the aerial view of google maps it looks closer to 75). That means your march would be 8.53 miles long full of people.
Now according to this, LINK
the march route is 1.1 mile long. Now granted everyone wasn't necessarily on the march route at the same time, but where there really 7 times as many waiting to start as were marching the entire length of the route?. IN order to get 8.5 miles, you would have to do the march route, go past the supreme court to Independence Ave, go all the way back down to the Lincoln Memorial, cross back to Constitution, go all the way back past the Capital again, go back down 1st to Independence again and continue until you were even with the Washington Monument (I used LINK
this for the mileage). 1.5 laps around the entire mall using independence and constitution.
Another way to look at is, starting from the rally site, walking about .2 miles short of the beltway heading North.
Only if these routes were completely filled with people packed pretty close together think about like it is when you are leaving a football game, would you have half a million people.
Anytime you here a crowd size estimate you should divide it by 3. It wouldn't shock me if the inauguration only had .5 million. Very few things really get crowds that big, people just base their estimates on the incorrect estimates they have heard in the past.