I think it's a hair density thing, not a hair thickness thing.
...however, since I have both high density and medium/thick strands, I can't relate nor confirm any of our theories here. All I know is that for me, hotel size shampoos are more like 'two-shower' sizes 'cause that's about how long they last. They're good for emergency washes when visiting somewhere, but I definitely couldn't make it for months on end with just one.
Bookmarks