I think there's more randomness than logic as far as what sells when and where and for how much.Yes I do similar sizes to yours 70g ($13) and 40g ($5) . I call them regular and 'sample'. I sell the sample ones at just over cost price in an effort to get people to try them.
Some days the stars align and bring out the right buyers ,and I have a great sales day. I make the error of believing I'm selling a desirable product at a good price. Then the next market comes along and I hardly sell enough to cover my vendor fee.
Other times I have a soap that just won't sell, so I relabel it with a new name and poof! it sells. Ya jest never know...
Maybe try calling your shampoos "bath" and "travel" size (or maxi and mini, or ....) and see if that makes any difference.