At FanGraphs, Eno Sarris did some digging and found that in 2016, for the first time since 1925, Major League Baseball saw only league-average production from its outfielders. He derived that by using wRC+, or adjusted Weighted Runs Created, a statistic that individually weights a player’s various offensive contributions, then adjusts for league and park effects. The offensive decline, Sarris finds, has a lot to do specifically with a decline in power, which is coupled with outfielders getting smaller and faster.
While we do have the stereotypical outfielder build in Mike Trout (6’1″, 235 lbs.) and Bryce Harper (6’2″, 230), we’re seeing lots of young players who defy that mold: Mookie Betts (5’9″, 156), Adam Eaton (5’9″, 180), Jackie Bradley, Jr. (5’10”, 195), Ender Inciarte (5’11”, 165), and Billy Hamilton (6’1″, 160), for example.
Sarris also compares current fourth outfielder types to those 30 years ago and finds that, indeed, clubs have eschewed power in favor of speed and defense. He suggests that this could lead to a market inefficiency in which these speedy, defensive types are overvalued and the slower, power-hitting types (like Brandon Moss) are undervalued.