Recent studies document a social-origins gap or direct effect of social origin (DESO) on labour market outcomes over and above respondents' education, challenging the idea that post-industrial societies are education-based meritocracies. Yet, the literature offers insufficient explanations on DESO heterogeneity across education and different labour market outcomes. Little is also known about underlying mechanisms. We contribute by answering two questions: (i) How does DESO vary when comparing college-degree holders with non-holders? (ii) For which specific parental and children's occupations is the largest DESO observed? We focus on Spain, using a large new dataset (n = 144,286). Firstly, we find a larger DESO on socioeconomic status among non-degree holders, and on income among degree holders. We propose the notions of compensatory advantage in occupational attainment and boosting advantage in income for high social-origin individuals to explain these opposite patterns, drawing from 'downward mobility avoidance' and 'effectively maintained inequality' theories. Secondly, we map origin and destination micro-classes where DESO is largest. High-grade managerial and professional parental occupations, characterized by social closure and influence in large organizations, are the origin micro-classes exerting the largest DESO. We also find that compensatory advantage for low-educated children from advantaged origins is related to their higher chances of accessing managerial occupations, while boosting advantage on income among college graduates is observed for high-grade managers and liberal professionals, suggesting that micro-class reproduction may partially account for boosting advantage. We conclude by discussing the generalizability of our findings to other countries and their implications for research on DESO, meritocracy and social mobility. [ABSTRACT FROM AUTHOR]