Re: That probably explains..
"A bigger battery won't overcome the technical degradation of lithium ion batteries. Also no one wants a heavier phone than needed"
The Sony Z5C is contemporaneous with the iPhone 6s, and has a similar size screen. The weights are the same within a few grammes. The Sony has a battery 50% larger than that in the iPhone.
Of course, putting in a bigger battery will, other things being equal, reduce the rate of wear and give a bigger operating margin.
Would you like to reconsider your post?