Quality of LiPo cells and construction methods. The Power 4 OEM batteries get just a bit puffy from heat during a flight, but it is the plastic name and information plates that warp.
The ones I have did that right from the start, but the cells are not puffing it is just the plastic covers on each side. If the cells are puffing, the frame of the battery will warp also and make installation and removal of the battery in the aircraft difficult at best.
The use of an aftermarket charger that measures IR of the battery cells is a real must for tracking and maintaining the health of your batteries.