Most people overdose on detergent–it’s not a nefarious plot or stupidity on consumers’ part; just how we are. Companies can count on selling us a little bit extra, and sales are actually falling slightly due to the growing popularity of pre-measured detergent pods. They might contain too much soap for a small load, but also remove entirely the ability to over-estimate the amount needed.
In this scenario, everyone wins. Walmart gets to stock its shelves with smaller bottles, giving them more shelf space to cram more merchandise on. They also get a little bit of enviromnental cred: using less plastic to move more detergent is a good thing. Consumers get to carry lighter bottles to their homes and/or cars, but there’s a disadvantage for us, too.
When detergent is more concentrated, that means that when we pour with a heavy hand, we use even more. The Wall Street Journal notes that overall detergent sales went up the last time major brands went through a “round of compaction” in 2008.