As the equity markets struggle for direction, market breadth is suggesting that it's a narrow group of names that is outperforming the broader market. Market breadth continues to deteriorate and has been trending lower over the last several years.The narrowing of market breadth is especially notable in large-caps as names like Apple, Facebook and Microsoft are driving this market higher. While mid-caps and small-caps haven't seen the equal-weight/market-cap-weight ratio decline quite as steeply, the descent in all three groups has picked up steam since the end of the first quarter.Not a healthy sign for this market if you're hoping for a rebound from here.