I agree with you on this. I also have the same feeling that the 7 plus body is not as sturdy as that of the 6s plus. When I twist the 7 plus without too much force, I can notice the phone flexing a little. When I stop twisting, the phone went back to a normal state.
What makes the creaking noise is that the aluminium body of the 7plus is much more flexable than that of the 6s plus. When the body is flexed, the front glass or even some internal components will also be bent.
I think Apple did this on purose because of the fact is that phones are under some kind of pressure in daily use, such as putting the phone in tight pocket or accidentally lying on it. So, rather than making the aluminium body sturdy and hard, which is difficult to bend but once it is bent then it will not flex back, Apple made the aluminium body a bit more flexible but not soft so that the 7 plus can undergo some rough conditions. This reminds me of the LG G Flex
As a result, the overal weight of the 7 plus lighter than the 6s plus, eventhough the 7 plus comes with bigger battery, dual cameras, extra component for the new 3d touch home button, a new speaker at the earpice and bigger taptic engine. So many components were added but removing only the 3.5mm headphone jack, which means more weight for the phone. The only way to maintain, if not reduce, the weight of the phone as much as possible is to strip off some weight in the aluminium housing.
That's all what I think, but I still prefer the sturdy built of the 6s plus to the 7 plus.