Large group size may increase aggression, decrease productivity, and impair animal welfare. The objectives of this study were to assess the effects of group size on social behaviour post-mixing, particularly the distribution of aggression, associated injuries, and resulting productivity. A total of 480 growing-finishing pigs were used in a 12-wk study to quantify the effects of group size (10, 20, 40, and 80 pigs per pen) on aggression, aggression-related injuries, behavioural patterns, and the relationships among aggression, injuries, and growth rate. Pigs were regrouped at 8 weeks of age. Aggression of pigs during the first 8 h post-regrouping was observed by live observation. Injury scores were assessed on individual pigs 48 h after regrouping. Pigs were weighed individually at the beginning and end of the study to determine average daily gain. Total duration of aggression for the 8 h post-regrouping was similar among group sizes and average approximately 14 min per pig. The total number of fights per pig in the groups of 10 pigs was lower than for group of 20, 40, and 80 pigs. When group size was increased to 40 pigs per pen, the average number of naïve fighting pairs (neither pigs had previously fought) increased significantly. However, the ratio of total pigs in the pen to the number of naïve fighting pairs was similar among group sizes. The longest duration of fights and most number of fights occurred in the first 2 h post-regrouping. The proportion of pigs that did not fight during the 8 h post-regrouping was higher in groups of 40 and 80 pigs. Pigs with similar body weights fought for longer time than pigs with large difference in body weight. Total injuries scores, which were positively correlated with aggression, were similar among group sizes. Although initial body weight was positively correlated with the number of fights, the duration of fights and total injury scores, the overall average daily gain was not correlated with aggression and injury scores post-regrouping. In conclusion, pigs in large groups did not evidence a high level of aggression. The commonly held fears of increased aggression, its consequent injuries, and decreased productivity in large groups were not found in this study.