Secondary (functional) mitral regurgitation (SMR) most commonly arises secondary to left ventricular (LV) dilation/dysfunction. The concept of disproportionately severe SMR was proposed to help explain the different results of two randomised trials of transcatheter edge-to-edge mitral valve repair (TEER) versus medical therapy. This concept is based on the fact that effective regurgitant orifice area (EROA) depends on LV end-diastolic volume (LVEDV), ejection fraction, regurgitant fraction and the velocity-time integral of SMR. This review focuses on the haemodynamic framework underlying the concept and the myths and misconceptions arising from it. Each component of EROA/LVEDV is prone to measurement error which can result in misclassification of individual patients. Moreover, EROA is typically measured at peak systole rather than its mean value over the duration of MR. This can result in physiologically impossible values of EROA or regurgitant volume. Although the EROA/LVEDV ratio (1) emphasises that grading MR severity needs to consider LV size and function and (2) helps explain the different outcomes between COAPT and MITRAFR, there are important factors that are not included. Among these are left atrial compliance, LV pressure and ejection fraction, pulmonary hypertension, right ventricular function and tricuspid regurgitation. Because medical therapy can reduce LV volumes and improve both LV function and SMR severity, the key to patient selection is forced titration of neurohormonal antagonists to the target doses that have been proven in clinical trials (along with cardiac resynchronisation when appropriate). Patients who continue to have symptomatic severe SMR after doing so should be considered for TEER.