Maybe someday, my good buddy, you will wrap your head around the concept that S&D is a VERY BIG structure that includes all the things you just mentioned within it. If you were talking about galaxies and quarks and black holes and dark matter, S&D is like the gravity that exists in the entire universe. It sounds like you are understanding many of the pieces of the puzzle, you just can't see the entire picture quite yet.

Production costs go up (Supply), effecting the prices, which will then effect Demand and thus quantities. Say there's a shortage of rubber (Supply), the price of products that use rubber goes up. Demand for those rubber products goes down as a result. (There is of course elasticity and inelasticity of Demand, but that's still Demand.)

Marketing convinces consumers they have to have some great product. Demand. Do you get what I'm saying? Marketing, and every other thing in the universe that effects a decision to buy something, is simply part of Demand. Shrinking a bag of chips will have some effect on Demand. Supply and Demand are not pieces of a puzzle, they are the entire puzzle. Everything else is a piece of that puzzle. There is nothing magical about it, nothing good or evil about, nothing innately moral or immoral about it. It just IS, like gravity.

Btw, I know I passed up my tapatio doritos when I first noticed the slightly smaller size, and I still only buy them when they are on sale, which is as often as not. Demand. In a competitive market some companies will keep their bag sizes the same because they want market share. But brand loyalty is a thing too though, and I just don't fucking like flaming hot Lays chips.