Abstract Extended social networks encompass both weak and strong ties to provide social support and resources. Hence, it is important to study what explains variation in these networks. This paper addresses this and examines the size and ethnic homogeneity of extended social networks, and group differences therein, and it aims to explain these differences based on a preference–opportunities approach through a decomposition analysis. We apply state‐of‐the‐art NSUM methods to measure the extended networks for different ethnic‐majority and minoritized groups, also considering migrant generation differences, in the Netherlands. Results show that group differences in network size reflect first‐generation minority citizens having smaller networks and majority citizens having more ethnically homogeneous networks. More positive out‐group attitudes among the Moroccan– and Turkish–Dutch partly explain why these groups have less homogeneous networks than Dutch majority members. Differences in the ethnic composition of neighbourhoods also contribute to explaining the homogeneity gap between Dutch majority and Turkish‐Dutch.