Here's a quote from The Stretching Roundtable.
Shugart: Okay, Don, you've written articles about stretching for size gains. How does that work?
Alessi: There are several ways this occurs. For one, stretching increases joint range of motion, which increases the distance that a load is moved thereby increasing the work performed. Also, specific methods like PNF stretching increases isometric and dynamic strength, muscular endurance, and functional flexibility.
Additionally, specific stretching conditions the stretch reflexes that are involved in weight training, thus shutting off these protective mechanisms. This allows the trainee to train at a higher percentage of his rep maximum more often. That means a greater force traveling over a greater distance.
Finally, stretching of the parallel elastic components (PEC) immediately after resistance training increases muscle hypertrophy by stretching the limiting ?sheaths? that encapsulate the muscle belly. In protective response to this unstable change, the stretched muscle sheets trigger an increase in protein splitting, muscle cell division, and collagen breakdown and repair. The result is hypertrophy or "thickening" for survival.