This paper analyzes contract efficiency with regard to correlated project realization and the size of the borrowers in group lending. Firstly, I show that under the standard assumption of independent project payoffs, the expected group cost of default decreases with group size. Secondly, I show that small groups can also optimize group efficiency if individual payoffs and credit risks are correlated. The results outline that social cost minimization occurs due to a common interest in forming optimal borrower groups between lenders and borrowers.