Previous health communication research has demonstrated the negative psychological and health effects of depicting thin-sized models in mass media advertisements including on social media sites such as Instagram. However, gym advertisements are one common source for the presentation of lean and thin-sized models on Instagram. Therefore, the current study guided by social comparison theory and signaling theory aimed to experimentally examine the effect of thin-sized models relative to plus-sized models as well as slogan-type (health and wellness versus physique-based) on women's appearance comparison, body satisfaction, perceived gym fit, and intentions to join the gym. A sample of 217 undergraduate students who identified as women were randomly assigned to one of four Instagram gym advertisement conditions varying in model body-size and slogan-type. Appearance comparisons, perceived gym fit, and intentions to join the gym were measured post advertisement exposure and body satisfaction was measured pre-and-post advertisement exposure. As expected, exposure to Instagram gym advertisements featuring thin-sized models resulted in greater appearance comparisons and lower body satisfaction than exposure to Instagram gym advertisements featuring plus-sized models. Moreover, the combination of plus-sized models with health and wellness slogans in Instagram gym advertisements resulted in greater gym fit perceptions although there was no effect of model body-size and slogan-type on intentions to join the gym. This study supports social comparison theory, signaling theory, and practically the findings indicate that Instagram gym advertisements that depict plus-sized models (versus thin) with health-and-wellness slogans (versus physique) generate fewer body image concerns and lead to greater gym fit perceptions.