design idea - Squash bottle

A thread on Core77.com discussed a new packaging design, where a consumer buys a spray pump with a bottle of concentrate, to which they add water. They then buy more concentrate as necessary. This cuts down on weight during transportation and saves throwing away the bottle. It got me thinking, if you could cut down on weight, you could also cut down on volume, making more environmental savings. I did a quick sketch for a collapsible bottle, something that has been done before, but not applied in this way because the volume always had to be 100%.
This started me thinking..... does reducing the volume of the bottle actually make any cost/environmental saving and if so , how much? Is there a tool to help designers easily calculate this? I couldn’t find one, so I’m now working on a calculator that I will put in the download section as soon as it’s finished.