It's a already know problem in game. The reason is it's ignore splash target defence and resistance; going for the 80% damage to be calculate directly on the hp. Or this problem has been resolve in china. They are mostly waiting for more powerfull ninja to be release before nerfing this effect. Tsunade swimsuit was going with that problem too. Dont know she got the same problem.

Right now it's mostly a exploit of a problem. Clone are calculate with the hp of there creator and strangely get no bonus or maybe the def/res they should have from there creater is not calculate in the damage formula; increasing by a lot the damage deal with splash attack.