The front low-angle picture is the one Trump fans use to "disprove" the story. Its angle hides the swaths of empty space that were filled during Obama's first inauguration. Not to mention that the low-angle front picture obscures the Obama 2009 crowd spilling out the sides.

The picture you linked was taken earlier in the day, but is not the one used in the comparisons on news sites. That picture was taken around the time the Obama 2009 one was taken and shows Trump's crowd slightly more filled in: http://i.imgur.com/17nLdRl.jpg

Obama's inauguration broke all records and Trump's didn't come close. The scary point is that Trump felt the need to lie about it. They even admitted it with their "alternative facts" line. Openly lying about things that are demonstrably false is his way of distracting from other issues.