In the world of money management, bigger is often considered better. College and university endowments greater than $1 billion, for example, have long outperformed their smaller rivals. That may be changing. In the latest annual National Association of College and University Business Officers-Commonfund study of endowment performance, the smallest endowments— those under $25 million — edged out the biggest endowments, averaging a five-year annualized return of 10.6 percent to the $1 billion-plus category’s 10.4 percent.