Despite its significance in determining poverty risk, family size has received little focus in recent social policy analysis. This paper provides a correction, focusing squarely on the changing poverty risk of larger families (those with three or more dependent children) in the UK over recent years. It argues that we need to pay much closer attention to how and why poverty risk differs according to family size. Our analysis of Family Resource Survey data reveals how far changes in child poverty rates since 1997 – both falling poverty risk to 2012/13 and increases since then – have been concentrated in larger families. Social security changes are identified as central: these have affected larger families most as they have greater need for support, due to both lower work intensity and higher household needs. By interrogating the way policy change has affected families of different sizes the paper seeks to increase understanding of the effects of different poverty reduction strategies, with implications for policy debates in the UK and beyond. In providing evidence about the socio-demographics of larger families and their changing poverty risk it also aims to inform contested debates about the state’s role in providing financial support for children.