It's not bagging, just fact that the following is true:

1-Big phones are harder to grip than small ones; therefore, most "phablets" have either a curved rear, a textured finish or both. The new iPhones have neither.

2-The iphone 6 plus is very wide--but has that flat back. That means you have no point of contact with the rear of the phone for extra grip – all force is on the sides. This, plus the super smooth finish, means its very easy for it to slip out your hands.