Good article with Jon Kaase on piston balancing.

Doug Rahn wrote:http://mooregoodink.com/clarifying-chief-misconceptions-of-piston-balancing/?utm_source=ActiveCampaign&utm_medium=email&utm_content=MGI+News+Brief+Issue+%2355&utm_campaign=MGI+News+Brief+Issue+%2355+Trade+-+no+bike

That is the main reason that production connecting rods break just below the pin.  When running, the rod is usually not centered in the piston, (front to back), so at overlap, the tensile forces on one side is greater than the forces on the other side which causes the rod to bend very slightly just below the pin and after a while it breaks.
Making one side of the piston lighter than the other can help some however, getting the rod-offset to place the rod pin end in the center of the piston is really what it takes.

A smaller offset of the rod pin end in the piston usually won't matter much at a lower RPM but, as RPM gets greater it will make more and more difference.