Objectives: The American College of Cardiology (ACC) guidelines recommend the same imaging frequency for all bioprosthetic valves, but some have demonstrated poor durability. We aimed to assess mortality differences between small (19–21 mm) and large (23–29 mm) in Mitroflow and Carpentier‐Edwards Perimount aortic valves.Methods: A retrospective observational study was conducted by all patients undergoing isolated surgical aortic valve replacement with Mitroflow or Perimount in Western Denmark between 1999 and 2014 and followed until January 2024. The primary endpoint was all‐cause mortality. Secondary endpoints were cardiovascular mortality and sudden cardiac death. A propensity score‐matched analysis was performed.Results: A total of 1150 patients were analyzed, with 496 (43%) receiving Mitroflow valves and 654 (57%) receiving Perimount valves. In the Mitroflow group, 108 (22%) had a valve size of 19–21 mm, and 388 (78%) in the size range of 23–29 mm. In the Perimount group, the distribution was 99 (15%) and 555 (85%), respectively. The compromised survival of Mitroflow valves was attributed to the valve type, regardless of the valve sizes. Larger Mitroflow valves exhibited the same compromised late mortality as smaller valves, 66.7% vs 61.5%, respectively (p = 0.95). The same pattern of mortality was observed in the matched population, with Perimount demonstrating significant lower risk of mortality.Conclusion: Mitroflow valves were associated with a poorer prognosis compared to Perimount valves. Additionally, larger Mitroflow valves were not associated with an improved prognosis compared to smaller valve sizes. EuroSCORE2 had a significant impact on patient survival.