Pupil size, as a window into the minds of others, plays a crucial role in social interaction. While previous studies have focused on the influence of non-social factors, such as the physical properties of stimuli, on pupil diameter, recent research has emphasized the significant connection between social information processing and pupil size. In this comprehensive review, we aim to explore how the processing of social stimuli (e.g., face, biological motion) and their emotional characteristics affect pupil size. In essence, pupil size is believed to reflect an individual’s perception of social stimuli. It goes beyond simple physical properties and encompasses the processing of complex social information, including social contexts and interactions. The modulation of pupil size in response to social stimuli is believed to be driven by two key mechanisms: emotional arousal and social attention. When individuals encounter emotionally charged social cues, their pupils tend to dilate, indicating heightened emotional engagement. Similarly, the dilation of pupils when individuals focus on specific social cues suggests an increased allocation of cognitive resources to process relevant social information. Furthermore, the connection between pupil size and social information processing has provided intriguing findings in individuals with autism spectrum disorder (ASD). Known for their significant social deficits, individuals with ASD exhibited abnormal pupillary responses when presented with social stimuli. These findings raise the possibility of utilizing pupillary responses as a potential index for identifying individuals with ASD at a relatively younger age. Moreover, the incorporation of pupillary response measurements in the diagnosis holds great promise in transcending the limitations of the minimum diagnostic age. This can have important implications both in terms of theoretical understanding and practical applications related to the diagnosis and intervention of ASD.