I like to think of the differences in the scope of each level of document. The Level 1 manual covers the entire organization, each Level 2 procedure tends to be owned by one department (although others are involved in the process defined by the procedure), and Level 3 work instructions tend to be very narrow ... such as maintenance instructions for a particular type of machine, calibration instructions for a particular type of gage, job description for a particular role, specification for a particular part number, etc. So while the scope narrows as you go deeper in levels, the number of documents grows. I once worked with an organization where we estimated they had between 50,000 and 70,000 level 3 documents (they were a low volume, high variety producer of a product that had many different components -- think of the number of drawings, routings, etc.).