Before revision:  I show that in both cases the coefficient on treatment is identical to a convex combination of two other parameters(different for OLS and 2SLS), which can be interpreted as the average treatment effects on the treated and controls under additional assumptions. Importantly, the OLS and 2SLS weights on these parameters are inversely related to the proportion of  each group.
After revision:  I show that in both cases the coefficient on treatment is identical to a convex combination of two other parameters(different for OLS and 2SLS), which can be interpreted as the average treatment effects on the treated and untreated. The weights on these parameters are inversely related to the proportion of each group.