I haven't seen the specific kits you mentioned but those I have seen are nice. However part of the reason they are at such an affordable price point is they don't have the level of detail of some of the other more expensive kits.
RoG produces products at various price points, knowing where their market strengths are and serves those customers. Tamiya on the other hand doesn't release kits for the beginner or even in most case the novice to intermediate level modeler, but the advanced modelers.
Cost is directly related to engineering of the molds used to create the product. The more intricate and greater the parts count, the more expensive the product will be. Size isn't a factor.