During the time of HSDS, Mo Dong Sect, despite being in operation for only a few decades, was already a major sect with hundreds (if not thousands) of members. Only the Mo Dong 7 Heroes, however, were heard to address Cheung 3 Fung directly as "teacher."
Does that mean that the vast majority of Mo Dong Sect students in HSDS were either Cheung 3 Fung's grandstudents or even great grandstudents, and only the 7 Heroes were his direct students? That seems sort of odd because at the beginning of HSDS, the oldest of the 7 Heroes, Sung Yeun Kiu, was only in his thirties or maybe forties, and the youngest, Mok Sing Guk, was only in his early twenties. If that's the case, then Cheung 3 Fung didn't start accepting disciples and heading the Mo Dong Sect until within thirty or so years of HSDS's main storyline beginning...allowing very little time for the sect to have grown as large as it did.