I believe it is based on the size of the object and these setting in the fpe file below: Add them if not there.
explodable = 0
debrisshape = 0
Try changing the debris shape to 1, and also try this on different size objects to see if it matches the shape.
This is a guess based on classic usage.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 4gb RAM, Nvidia gtx660, Windows 7 Pro 64bit