As spring brings new buying opportunities to the market, with the S&P 500 Index ($SPX) pulling back from the new all-time highs set in March, Jefferies sees promise in mid-cap stocks. While big players dominated last year’s rally, mid-caps fell behind. Mid-caps were sized at 10% of the U.S. equity market, falling short of the historical average of 15% since 1926. However, Jefferies strategist Steven DeSanctis believes mid-caps look ready to make a comeback, as he predicts their earnings growth will pick up speed in the second half of the year, boosting relative performance. Among the overlooke…