Ah, I've obviously spent too long working with databases...
Low granularity means a lot of data is lumped together in one chunk, so a 'Guardian crossword' thread (covering every Guardian puzzle, ever) would simply grow and grow as each new Guardian puzzle came out. 'Guardian 27,756' is more granular, as only data (posts) relating to that particular puzzle will be grouped together. 'Guardian 27,756 7dn' would reflect a highly granular structure, as only posts relating to that clue will be grouped under that heading. Essentially, greater granularity = more levels of detail.