Costing is a very complicated business. It's not a case of a bigger size using more material therefore being worth more. Sizes are cut in size runs and they are scaled, based and priced around the median/most popular size. In the amount of material that covers two medians, you also get the size that's median minus one and one that's median plus one. And median plus two and median minus two. This is why sizing isn't based on specific measurements, but scale around the most common size. And that's before we even look into design and all the other zillions of things that have to happen to produce clothes. And then costing takes all of this into account in what to charge for the item.
It's really, really complicated and apart from being a marketing nightmare, making smaller clothes cheaper would just add an extra layer of complication. Bear in mind that many smaller sizes are cut only because the corresponding larger sizes are being made and there's no point in wasting material; that would cost even more.
With kids' clothes, the size breaks will be different because children's proportions change as they grow up in a way adults' don't as they get into bigger sizes, and also because lots of styles for very young kids won't be made for older kids as they won't find them appealing. The size runs can therefore be more specific to themselves, but the whole thing is still really complicated.
Edited because I misread the kids' clothes question.
Edited again to explain that the more sizes a range comes in, the more size runs you'll need because there comes a point where the proportions stop working and you'd start making clothes for actual giants or midgets, so you need a size break and a new run with a new medium tl keep the proportions correct as people get fatter or thinner. Obviously this happens more for growing children so the size runs are more internally consistent.