I would think that bigger stadiums quickly become smaller home field advantages. Simply put, when a team is winning, the seats will mainly be filled by that teams fanbase, but if a team starts playing mediocre or poorly, a large stadium will attract the opponents teams fanbase because more seats are available. Add to that DC's proximity to a lot of other football fans cities (2hrs to pitt, NyJ,NYG, and Philly, a little more for Carolina), plus DC's own weird mix of allegiances and you get a stadium that will easily be overshadowed by other fans. (It was interesting to hear the Saints fans take over the new Cowboys stadium at points on Thanksgiving day
).
If the Skins field a consistent winning team FedEx will rock, until then, homefield is just another stadium.