It's a matter of the shape and size of the magnetic fields. With a standard/wide spaced magnetic chuck the magnetic fields extend well above the top of the chuck surface. Large loops. With a fine magnetic chuck they do not extend very far above the surface, so only very small parts are really "encompassed" by the fields. In most situations large parts are held just fine by fine pole chucks, according to my reading (with zero actual experience with a fine pole chuck, and minimal with a standard; just relaying the theory), because the parts are solid and therefore holding the bottom is sufficient. But based on my understanding of magnetic fields, magnetic chucks, and leverage, I wouldn't want to use a fine pole chuck on a part near the vertical capacity of my grinder.
If you have the budget for it, I recently saw that Suburban Tool at least used to make one that had alternating coarse and fine poles. I believe Tom Lipton showed it (used) in a video on his Oxtool YouTube channel. That strikes me as an ideal design.
