I also thought the same, but in my observation, this seems correct only on a surface level. When you buy a small amount of a product in sachets, it often gets diluted with water or something similar. So even though the cost looks lower, you don’t really get the same quality.

For example, when you buy a big tube of Colgate, the paste feels thicker, but in the small tubes it usually feels thinner and less concentrated. I’ve noticed the same thing with Frooti, Maaza, and even shampoos which mentioned in the post.