What makes certain zeolites better at ion exchange? I would have thought that the bigger the pores in the zeolite, the more ions it can contain, and therefore the better the use of the zeolite for ion exchange.

However, zeolite A has small pores but its main use is as an ion exchanger, as opposed to many other zeolites which have larger pores but are not used as ion exchangers.

Therefore, why are some zeolites better than others at ion exchange? Or, what influences the ion exchange capabilities of zeolites beyond pore size?
Zeolites are selective ion exchangers.

What is usually required for a good zeolite is that it selectively absorbs the ion you care about rather than any old ions in the mixture.

That selectivity is a product of the shape of the holes in the 3D structure. Bigger holes will be less selective than smaller holes. If all the ions in the mix fit in the hole the zeolite won't reject the ones you don't want. So, ideally, the holes will be big enough for one ion but too small for another one. This allows selective exchange.

That is about it. It isn't about general equilibration of ions; it is about selective exchange of ions.