It's about a weight of the device and its portability, you can hold a smaller one easily in one hand. A typical ereader size is about 6'' and it weighs less than a usual paperback. Amazon had 10'' Kindle, but it wasn't very popular. So it's about a comfort of reading. Another issue is LCD vs eInk screen. eInk is optimised for reading, it's B&W and it gives an illusion of reading a paper page. LCD is backlit and some don't mind it and some do when they use it for longer. So depends what you want, when reading then you are probably better off with a dedicated eInk ereader, when reading is only a part of it, get a tablet. Its size will influence your comfort using it.