Because demand curves don't work like that unless you have a highly inelastic good. With normal goods keeping quantity constant as price changes puts you on a suboptimal point on the demand curve. Coffee's price certainly hasn't been inelastic and changing can size at every spike would have netted you decent production cost increases instead of savings: http://www.aboutinflation.com/_/rsrc/1368022029715/coffee-vs...
The varying of quantities in the same sized container is not the problem and is certainly not illogical. The problem is when we allow marketing departments to lie/cheat/fool the product onto a different portion of the demand curve instead of letting the $/amount naturally flow back when price decreases or when it makes sense to switch to cheaper containers.
The varying of quantities in the same sized container is not the problem and is certainly not illogical. The problem is when we allow marketing departments to lie/cheat/fool the product onto a different portion of the demand curve instead of letting the $/amount naturally flow back when price decreases or when it makes sense to switch to cheaper containers.