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.