One answer is the Dutch papermakers first invented the two-sheet paper mold in 1690. Legend has it that the average maximum reach of an experienced vatman’s arms was 44 inches. Many paper molds during that era were around 17″ front to back. This was because the laid lines and watermarks had to run from left to right. As a result, the size was limited the two full sheets of paper.
Before World War I, many industrialized countries started discussing the advantages of standardizing paper sizes to make it easy for their post offices to handle mail. Most accepted the ISA standard sizes in 1925. This is where the A6 paper size came from. Post World War II, The USA Government held a joint meeting between paper manufacturers, distributors, and users and drafted the Simplified Practice Recommendation R22. This formalized all the existing sizes most frequently used and stopped the addition of new ones. Simplified Practice Recommendation R22 became effective June 15, 1933.
So next time you pick up a printout, think about what it would look four times larger. Now you know why your paper is that big.
Based on an article by Institute of Paper Science & Technology