Wide cross-country variation in obesity rates has been reported between European Union member states. Although the existing cross-country differences have not been analyzed in depth, they contain important information on health production determinants. In this paper we apply a methodology for conducting standardized cross-country comparisons of body mass index (BMI). We draw on estimations of the marginal density function of BMI for Italy and Spain in 2003, two countries with similar GDP and socio-economic conditions. We produce different counterfactual distribution estimates using covariates (health production inputs) specified in a quantile regression. Our findings suggest that Spain-to-Italy BMI gaps among females are largely explained by cross-country variation in the returns to each covariate, especially for younger women. We find that adverse underlying determinants do not explain the gap observed in particular between younger Spanish females and their Italian counterfactuals; behavioural differences appear to be the key. We tentatively conclude that Spanish policy on obesity should target mainly younger females.