Forbes’ annual list of America’s Most Successful Mid-Cap companies includes several businesses that have more than doubled this year and several others that could be primed to break out. Growth stock investor Alger Management points out that mid-cap stocks as a group now trade at a 29% discount to large caps, bucking a historical trend of trading at a 2% premium on average. If those valuation disparities narrow, a boom for forgotten mid-cap portfolios could be around the corner.