Instagram is increasingly used in advertising, yet little is known about the unintended consequences of Instagram advertising on women’s and girls’ body image. Also largely unexplored is if and how curvy models (large breasts and buttocks, wide hips, and small waist) used in this advertising affect women’s and girls’ body image. We drew on social comparison and cultivation theories to explore if exposure to thin and curvy models through Instagram advertising is associated with late-adolescent girls’ willingness to take action to be thinner or curvier, respectively. Two mediation models examined the mechanisms through which any such effects occur. A sample of 284 17-19 year old girls completed self-administered online questionnaires. Results revealed that exposure to thin and curvy models was positively associated with willingness to take action to be thinner and curvier, respectively. These associations were mediated by thin/curvy body preference (model 1), and by thin/curvy body preference, upward physical appearance comparisons, and body dissatisfaction (model 2). Results suggest that although exposure to different body types may be associated with different types of unhealthy (body-altering) actions, the processes underlying these effects are similar. This research highlights possible cultural shifts toward more diverse body ideals and informs tailored body concern interventions and media literacy programs.