I dont get it to be honest. The plastic back on the 4a is perfectly fine. I have a cover anyway, who cares how it looks. I don't think it's more durable either? Why are plastic ones getting heavier though?
Probably packing more into the same space. Battery capacities have been going up and that's weight. Phones are getting bigger too. A few mm here and there and it's an extra 30% in volume, which also translates to more mass.