Studies suggest looming motion represents a special class of attentional capture stimulus due to behavioural urgency: the need to act upon objects moving toward us in an environment. In particular, one theory suggests that faster reaction times to targets cued by looming relative to receding motion are driven by post-attentional, motor-priming processes beyond the attentional capture effects seen with other stimulus qualities such as colour pop-out. The present study tested this theory using a relative size judgment task where targets were precued by looming and receding optic flow fields. Results show systematic increases in the perceived size of targets that were cued by looming flow fields, consistent with previous attentional capture studies using onset cues. These results challenge theories attributing behavioural changes from looming motion to motor-priming alone.