By Dan Weil Small-cap stocks began May about 21% undervalued, compared with 12% for large-cap stocks, according to Morningstar. The Russell 2000 index of small-cap stocks has dropped 22% this year, and that may have created some buying opportunities. Small-cap stocks began May about 21% undervalued, compared with 12% for large-cap stocks, according to Morningstar’s measures. “Given that small-company stocks have underperformed large-company stocks so far this month, small companies as a group remain the better bargains today,” wrote Susan Dziubinski, director of content for Morningstar. “We en…