As the size of the boat gets larger and heavier, you need to have more sail surface for the wind to interact with - the sail has to be larger. At some point, the sail gets just too large to handle. The solution is to add more sails that are smaller and easier to handle but collectively make up a large sail area. The second part of your question has to do with a phenomenon called apparent wind. Apparent wind is the wind you feel on your face as you move forward.
True wind is the wind that is blowing naturally.
That is because your forward motion is creating its own wind. Generally these are fast catamarans and iceboats, although some racing monohulls may be able to achieve this. The apparent wind is the wind that the boat sails in. This is because you can trim the sails so that the wind flows over them to create a lift, much like an airplane wing, that propels the boat. As you can see, there is a positive force against the inside of the sail, and a negative force pulling the outside of the sail.