There's something to be said about short posts. Sometimes, when you don't feel like slogging through a hundred words just to hear one argument or another, a short read looks a lot more attractive than some stodgy long essay.
The situation isn't the same from a writer's point of view, though. I suspect that a lot of us just sit down and let the words flow out of our heads, piling onto the computer screen in row after row of text. In short, I don't think that most of us are conscious about the length of our writings -- and by the time we wrap up a piece and see that it has gone far longer than we intended, our first instinct definitely does not involve cutting it down in size.
Sometimes it's a thinly-veiled method to mask our own insecurities. Not all of us find it easy to make a good point from the very beginning, so we end up spouting any number of random arguments until we do hit the right nerve. Empathic authors somehow manage to do this while sounding completely organized and believable. Other authors might end up offending people with some of their scattershot offerings.
I'm not sure when I stopped writing brief excerpts and starting writing the truly long stuff. It could be that I wanted to squeeze as many points into my work as possible -- this, in fact, would have resulted in some really thick paragraph blocks if it weren't for my penchant for standalone text fragments.
Or, if I were to take the reason given above -- I'm probably more insecure than I look.
Now there's a comforting thought.