Objectives <p>Body weight is still the most common metric used for donor-recipient matching in Pediatric Heart Transplantation (PHTX) and its impact on long-term outcomes remains unclear from variable definition and conflicting evidence. In this study, the outcomes of varying weight mismatch on PHTX was analysed.</p> Methods <p>The United Network for Organ Sharing (UNOS) database (1984–2025) was retrospectively analysed. Size mismatch as a percentage donor-recipient weight difference was categorized as Mild (≤ 20%), Moderate (20–30%), Extreme (&gt; 30%), and stratified as undersizing or oversizing. Demographics, clinical characteristics, and post-transplant outcomes were compared. One-year and 15-year mortality were analysed using multivariable logistic and Cox regression, respectively.</p> Results <p>Oversizing (80%, 9,175/11,583) was more common than undersizing (20%, 2,408/11,583). With increasing oversizing, the recipients were younger, had congenital heart disease (CHD) diagnosis, and elevated pulmonary vascular resistance but with shorter wait-list times. Oversizing had no impact on 30-day, 1-year, or 15-years survival. With increasing undersizing, recipient age was similar, had less restrictive cardiomyopathy diagnosis, and had female donor but with shorter wait-list times. Undersizing had inferior survival till 5-years but not at 15-years. However, on multivariable analysis, undersizing was not predictive of inferior survival anytime as was congenital heart disease diagnosis, Extracorporeal Membrane Oxygenation (ECMO) bridge, elevated pulmonary-vascular resistance, transplantation era, and post-transplant rejection, stroke, and dialysis use.</p> Conclusions <p>Oversizing is more likely than undersizing in PHTX. While oversizing had no overall survival impact, undersizing had negative mid-term but not long-term survival impact which disappeared when adjusted for underlying cardiac diagnosis, elevated pulmonary vascular resistance, transplantation era and occurrence of post-transplant complications.</p>