I read on Thorlabs' website that the coupling efficiency of a fiber is primarily dependent on the core diameter and numerical aperture of the fiber. It makes sense to me that if the beam of light going into the fiber has too great of an angle, the outer parts of the beam will fail to couple into the core of the fiber - which is why we need to make sure that the NA of the beam is less than or equal to that of the fiber. I don't understand though why core diameter affects coupling efficiency. It seems to me that fibers with different core diameters but equal NA will funnel the same amount of light because the NA angle is measured from the axis of the fiber so the allowed cone of light should be the same regardless of core diameter. Why exactly does core diameter affect fiber coupling efficiency?
On a related note, I've also read elsewhere that a larger diameter fiber with a lower NA has lower coupling capability than a smaller diameter fiber with a higher NA. Can you explain how that's possible?