John Hancock's pronounced signature on the Declaration of Independence has made his name a synonym for "signature", and there's a couple of legends as to why his name is signed so much larger than everyone else's. Some have said it was a willful challenge to King George III, others that he was daring British sympathizers to make him their first bounty. The real reason, it turns out, is very simple: Hancock's signature was the only one on it for nearly a month.
When the Declaration of Independence was adopted on July 4, 1776, John Hancock was president of the Continental Congress which adopted it. His signature authenticated the document before it was sent to the thirteen colonial legislatures for approval (and never directly to George I). His signature was attested by Charles Thomson, secretary of the Continental Congress, and printed copies were made and sent out.
Some of these printed copies still exist in the National Archives, and it can be plainly seen that John Hancock's name is the only one attached. The rest of the delegates would not sign the original Declaration until August 2, 1776.