This study investigated how university students behaved and interacted as they were grouped in large and small groups and while they engaged in intra-group and inter-group peer assessment. One hundred forty five students from two classes at a university in Ho Chi Minh, Vietnam participated in this one-term study. The students worked in mixed-gender and heterogeneous achievement groups. The results show that when the students worked in small groups of 4-5 students, every group member had a better chance of being involved in discussions and interactions, providing a better opportunity to improve their cognition than when they worked in large groups of 8-9 students. The findings also show that the students worked more effectively and felt more relaxed when they engaged in inter-group peer assessment than when they engaged in intra-group peer assessment. Future research should investigate similar strategies to make cooperative learning more adaptive in a non-Western context. (Contains 6 tables and 1 figure.)