The effect of prey size on the timing of the startle response in the sculpin Leptocottus armatus was investigated. Escape responses were triggered visually by a looming image obtained using a computer‐generated animation of an approaching black disk. The results showed that apparent looming threshold (TAL, i.e. the threshold at which the rate of change of the visual angle subtended by predator frontal profile onto the prey's eye triggers an escape response by the prey) decreased with increasing prey size. Distance travelled within a fixed time was unaffected by size. Theoretical considerations suggest that larger prey would need to travel a longer distance (and so they would need more time) in order to move their whole body outside the predator's approaching gape. Therefore, the scaling of TAL may be explained by taking into account both ultimate and proximate considerations that need not be mutually exclusive. At an ultimate level, lower TAL in larger fish may be explained in terms of offsetting the disadvantage of offering a larger volume to be intercepted by the predator. At a proximate level, TAL may be related to the fish's visual acuity, which is higher in larger fish.