It can often be the case that small increase in the diameter of a cork stopper can dramatically increase the price – this is particularly the case for natural cork, but is also an issue for agglomerated cork and this is why.
For natural corks, diameter is the most crucial parameter because of how the corks are made: to make larger diameter corks you need to have thicker cork bark, which means the bark has to be left on the tree for longer, which makes it much more expensive. Natural corks are made by punching through the cross section of bark and to make corks with a larger diameter than the standard 24mm generally means that the bark has to be left on the tree longer than the standard 9 years. This means that large diameter natural corks (particularly of high quality) rise in price exponentially with their diameter.
There is also an issue with agglomerated corks – all of the mid to high quality agglomerated corks are made using compression moulding and the mould size is generally a maximum of 32mm. Corks from these moulds can be ground down to size and shaped according to specification, so it is not a problem to make agglomerated corks with any diameter less than the maximum; however, in order to make a cork stopper or for example a one piece agglomerated bar-top cork that has a larger diameter than 32mm, this generally means that they have to be punched out of cork board, which is a much more time-consuming process.
Making smaller natural cork stoppers (typically bar-top corks for spirits) can be made less expensive by using rejected wine corks that can be ground down to size. A lot of wine corks will be rejected because of imperfections on their surface, but they can be used by grinding them down further to make smaller corks.
So staying within the standard parameters for the diameters of cork stoppers can save you a lot of money – when choosing a bottle for your spirits it can really be worth considering the mouth diameter if you are planning to use natural cork as you can potentially save a lot of money on your corks by choosing a narrower mouthed bottle.