"I have only made this letter rather long because I have not had time to make it shorter."
--- Pascal, "Lettres provinciales," 16, Dec.14,1656. Cassell's Book of Quotations,London,1912. P.718. "Je N'ai fait celle-ci plus longue que parceque je n'ai pas eu le loisir de la faire plus courte." Source. See also link.
"Please forgive this long letter, I didn't have time to write a short one."
--- George Bernard Shaw, who used to write 10 letters per day, who after writing a 50-page letter to someone, link