Male-male conflict is common among animals, but questions remain as to when, how and by whom aggression should be initiated. Factors that affect agonistic strategies include residency, the value of the contested resource and the fighting ability of the contestants. Game-theoretical models often assume that strategies for aggression are conditional and shaped by mutual assessment. We quantified aggression in a fish, the Australian desert goby, <i>Chlamydogobius eremius</i>, by exposing nest-holding males to male intruders. The perceived value of the resource (the nest) was manipulated by exposing half of the residents to sexually receptive females before the trial. We found resident male aggression to be unaffected by perceived mating opportunities. It was also unaffected by the size of the intruder. Instead, aggression was related the residents' own size, namely, smaller males attacked sooner and with greater intensity than larger males. Thus, contrary to theory, resident desert goby males appeared to have set strategies for initiating aggression. Rather than viewing high aggression in small males as a paradox (i.e. the Napoleon effect), we suggest that small individuals may benefit from attacking early, before an intruder has time to assess the resident and/or the resource.