Simpson's paradox refers to the fact, when comparing two populations with different subgroup size, the average of some variable in the first population can be better in every subgroup and yet worse in the total population.
Simpson's paradox refers to the fact that, when the size of subgroups depends on the treatment, the mean impact of a treatment can be positive in every subgroup while the mean impact is lower for the global population. A treatment can be better in every subgroup and yet worse for the total population.