Few studies have examined how parasite impact is affected by host size. In a glasshouse experiment, we investigated the impact of the Australian native hemiparasitic vine, Cassytha pubescens, on a major invasive shrub, Ulex europaeus, of different sizes. Infected plants had significantly lower total, shoot and root biomass, but the parasite's impact was more severe on small than large hosts. When infected small but not large hosts had significantly lower nodule biomass. Irrespective of size, infection significantly decreased host shoot/root ratio, predawn and midday quantum yields, maximum electron transport rates and carbon isotope composition, and host nodule biomass g-1 root biomass significantly increased in response to infection. Infection did not affect host foliar nitrogen concentration or midday shoot water potential. Parasite biomass was significantly lower on small relative to large hosts, but was similar g-1 host total biomass. Parasite stem nitrogen, phosphorous and potassium concentration were significantly greater when C. pubescens was growing on small than large hosts. Our results clearly show that C. pubescens strongly decreases performance of this major invasive shrub, especially when hosts are small. This suggests that C. pubescens could be used most effectively as a native biocontrol when deployed on smaller hosts.