What are the pros and cons of doing a PhD in a small research group compared to a large research group? I have chosen to work under a associate professor as my PhD advisor. He has a small research group -a group of 3 graduate students and some Undergraduates. I see colleagues of my advisor have groups of 10-12 students with 3 postdocs. 

Am I missing something- research wise ??
The individuals involved are more important than the size of the group.  Different faculty have different management styles.