I have talked to Brass importers about similar issues. I personally like smaller engines like consolidations and switchers and I was asking why they didn't make more of them. The importer explained the cost difference for similar quality in pretty minimal. A relatively small amount of material makes teh difference between a Big Boy and an 0-6-0. So he siad the costs to design and build are almost the same for the small one. However, buyers expect a smaller 0-6-0 to be vastly cheaper and therefore won't buy when only 10% or so less that the big articulated. Therefore he generally offers larger locos because they sell.
Also, the size of the run is very important, especially when new tooling is needed. the cost of the tooling needs to be amortized over the number of locos. If the number is small, say a few hundred, the amount each one must cover is much higher. If the run is 1000, the cost of the tooling and design per unit is much much lower.