Not an economist, but yes, the principle of “economies of scale” means that whatever you produce the most of will be cheaper than what you don’t make a lot of, because your infrastructure and supply chain will naturally be geared towards whatever your most common product is, its supplies can be bought cheaper in bulk, etc.  Making things outside the norm will cost the producer, and thus the customer, more.  If you are making 5,000 3X shirts and only 50 small ones, the smalls will probably thus be more expensive.