This study examined 3- to 5-year-old children's understandings of increases in body size via a qualitative prospective approach. A sample of 259 children (55.2% girls) was interviewed at 3, 4, and 5 years old. Participants were shown an average and a larger size figure of a child of their gender and age. Responses to 'Why do you think the boy/girl got bigger here?' were coded using thematic analysis. Diet was cited as a mechanism for increased body size by almost 50% of children referring to this by age 5. Few children mentioned physical activity. Responses suggesting that increases in body size had negative implications increased between ages 3 and 5. Awareness of associations between diet and weight gain emerges as young as 3 years old and increases over time, as do negative attitudes about weight gain. This age may be opportune for interventions targeting sustainable healthy behaviours. Statement of contribution What is already known on this subject? Even very young children present weight bias. Their understanding of the social messages regarding weight and shape increases with age. Older children present a partial understanding of how diet and physical activity are related to body size. What does this study add? Very young children's understandings of increases in body size were investigated. Eating and food-related mechanisms, as well as ageing, were salient. By age 5, over a third of children associated increases in body size with negative implications.