Looking at gsmarena site,
iPhone 5s, 5c, 6, and 6 plus,
all of them only have 1GB of RAM.
But, 6 and 6 plus, have a bigger resolution. With 6 plus having the biggest one.
Because of that, I got a feeling you are left will less RAM than 5s and 5c to use for your game.
Have you tried limiting your memory usage per scene using Atlases?
Alternatively, try removing the 4x scale (if you are using them).
Just let the 6 plus to load the 3x assets, instead of 4x.
You can barely notice the difference between 3x and 4x playing it on iPhone/iPad.