Reducing obesity and weight gain, which often occurs during breast cancer treatment, may represent an efficient secondary or tertiary prevention against cancer. This retrospective observational cohort study aimed to assess the impact of a Mediterranean diet on weight and anthropometric changes in women completing active breast cancer treatment. Additionally, we sought to identify factors associated with study dropout within one year. A total of 182 female patients (20 normal weight, 59 overweight, 103 obese) received personalized Mediterranean diet interventions and underwent monthly outpatient visits. Dropout rates were 42.3% at 6 months and 64.1% at 12 months. Among the obese subgroup, BMI (p < 0.001) and fat mass (p < 0.05) decreased after 6 months. At 12 months, the obese subgroup showed a borderline significant further reduction in BMI (p = 0.062). BMI or weight loss did not predict dropout at any time point. However, age (OR = 0.91) and diastolic blood pressure (OR = 1.07) were significant predictors of dropout at 12 months. Implementing a Mediterranean diet can lead to weight and anthropometric improvements in breast cancer survivors. Further research is necessary to explore the long-term effects of weight loss on these individuals, identify effective dietary approaches, and consider specific predictors of dropout.