Past studies on small group housing has found negative impacts of crowding on welfare. It has also been suggested that pigs in large groups are able to use space more efficiently. However, there has not been much research done on crowding in large groups. This study assesses space requirements for both small and large groups of pigs. The effects of space restriction on pig performance, behaviour, physiology, health and welfare was evaluated. Overall, crowded pigs had a lower growth rate and lower final body weight than uncrowded pigs. During the final week of the study growth rate was depressed 9.8%. Pigs housed in large groups experienced a 5.4% growth depression during the first 4 days after group formation. Interestingly, the first sign of depression in response to crowding occurred much sooner in large groups than in the small groups. However, the rate of depression of gain was more gradual in the large groups, therefore, final weights were similar for both small and large crowded groups of pigs. Overall crowded pigs had lower feed efficiency than uncrowded pigs. The level of crowding did not seem to have an effect on injury scoring or need for medical treatment. Group size did seem to affect injury scoring since pigs housed in large groups experienced a greater severity of lameness and leg injuries. Both crowding and large group housing were found to have negative impacts on pig performance. There was also little evidence to support that pigs in large groups were able to use space more efficiently than pigs in small groups.