By volume, you use more conditioner each hair wash than shampoo. So why, for the love of fluffy kittens, are the bottles sold the same size?

I hate starting the new conditioner bottle before the shampoo one, because it throws out my shopping planning, but if I start a new shampoo each time I start a new conditioner, I am left with lots of not-empty shampoo bottles.

Why, people who design packaging, why?!