Intercropping, i.e. the cultivation of crop species mixtures, can potentially reduce pressure on land resources by generating higher yields through exploitation of complementarities between crop species. Although intercropping is practiced on a non-negligible proportion of China's arable land, little is known about the factors that influence farmers’ decisions to use intercropping. In this study we develop a theoretical framework that distinguishes exogenous factors from endogenous factors in farmers’ activity choices in general and the use of intercropping in particular. We apply this framework in an empirical analysis of socio-economic factors affecting the use of traditional and novel relay intercropping systems, with a special focus on the impact of farm size, using primary data collected among 299 farmers in Gaotai County, northwest China. We find that large farms do no not plant more land with the traditional wheat/maize intercrop as compared to small farms, while land planted with the two novel intercrops is significantly larger on large farms. Availability of machinery has no negative effect on the area under intercropping, and has a significant positive effect on the use of one novel intercrop type. Our results confirm that risk considerations do not play a role in relay intercropping use decisions of Chinese farmers. We conclude that positive yield and natural resource effects of intercropping can still be realized if the ongoing farm scale enlargement policy is combined with a policy promoting novel intercropping types, particularly those types that can make use of already available machinery.