I've read that at least one reason these Chinese ships were so large was that the crews of such ships lived on board the vessels. They literally were born, married, and died there. As you might expect from the Chinese, many of the ships even had gardens on board. The Yuan (Mongol) dynasty of the 13th and 14th centuries was known to have trading ships large enough to accommodate sixty individual cabins for merchants and up to 300 crew members. Often these ships would go out and not return until a year or more later. These large ships were probably ideal for trading, but not so ideal for warfare as navy vessels.