Some confusing sentences on binding energy

I’ll paste the exact lines of my textbook:
(I) For nuclei of intermediate mass number 30<A<170 the binding energy per nucleon is almost constant.
(II) For light and heavy nuclei the binding energy per nucleon is lower.
From the above observations we can draw the following conclusions:
The almost constant value of binding energy in the range with intermediate mass number says that nuclear force is short-ranged. (How?)
If we increase A by adding nucleons they will not change the binding energy. Since most of the nucleons in a large nucleus reside inside it and not on the surface the change in binding energy per nucleon would be small.(What role does the position of nucleons play here?)
Thanks in advance.