Via 
Via “I have only made this letter longer because I have not had the time to make it shorter.”
—Blaise Pascal, (1623-1662) Lettres provinciales.
I’ve been looking for the source of this quote for a long time and now, via esquareda, this page provides what looks like a pretty thorough collection of the variations and original sources. Here are some of the others:
“Not that the story need be long, but it will take a long while to make it short.” —Henry David Thoreau
“If I had more time, I would have written a shorter letter.” —Marcus T. Cicero
“You know that I write slowly. This is chiefly because I am never satisfied until I have said as much as possible in a few words, and writing briefly takes far more time than writing at length.” —Karl Friedrich Gauss (1777-1855)
“If you want me to give you a two-hour presentation, I am ready today. If you want only a five-minute speech, it will take me two weeks to prepare.” —Mark Twain
Via 
