This study attempts to examine how students participate and interact in different discussion modules organized with different group size in an online environment. It adopts a case study methodology where full semester online course with two small-group and three class-wide discussion forums was examined. The researcher counted the number of messages and hits (i.e. the number of students' clicks on other messages) and analyzed that to classify the level of interactivity of every individual message into four categories, i.e. independent, quasi-interactive, interactive (elaborative) and interactive (negotiating). It found that the class guidelines and active encouragement by the instructor had contributed to the high number of total messages and hits representing students' overall participation in all discussion forums. However, large discussion forums in the course had their limitations in promoting higher level of interactivity among the students. On the other hand, two small-group discussion forums had a higher number (by 21%) of enhanced interactivity (elaborating and negotiating mode). The study then concludes that a high quality participation in a large online class could be effected through sub-grouping. It further alludes that other practices should be taken into consideration to promote interactivity in discussion. (Contains 3 figures and 3 tables.)