I've no idea of the Geography you are referring to in the DC area, being European. That said Uusimaa is a region, Helsinki is a (capital city). Helsinki spans quite a large geographic area and is fairly densely populated but primarily in a metropolitan area. Take a look on google maps and you'll see it's tiny, say compared to London, Paris, Berlin etc. Uusimaa covers a much bigger area with huge amounts of space between the edge and the metropolitan area (mostly covered with fields, private land and forest).