In a study of the spider Pholcus phalangioides, we double-mated large or small females with large and small males in all possible combinations to investigate the effects of body size and female mating history on male copulatory behaviour and paternity success. In addition we recorded data on female struggle behaviour. Our aim was to get insights into evolutionary forces that promote and constrain adaptive plasticity in male copulatory behaviour in this species with last-male sperm priority. When mating with virgin females, small males made more movements with both inserted pedipalps (sperm transfer organs) than large males, and males of both size classes showed many more pedipalp movements (PPMs) in large, fecund females. With mated females, in contrast, matings were either very short (5 min, 82% of matings), during which PPMs were little affected by male and female size, or as long as matings with virgin females (78 min, 18%) with comparably strong size effects on PPMs. Genetic paternity analysis showed that the second male fertilized on average 88% of the eggs, independent of his size and behaviour. Variation in relative fertilization success was solely predicted by the first male’s behaviour. We suggest that this difference in fertilization returns between first and second males may to a large extent explain the contrasting behavioural patterns in virgin versus mated females. The fact that female struggle behaviour contributed to variation in male copulatory behaviour further highlights that females are not merely passive during mating.