Obesity poses technical challenges during veno-venous extracorporeal membrane oxygenation (VV-ECMO) cannulation because of thick subcutaneous tissue and increased insertion resistance. While patient-related factors are well recognized, the impact of cannula geometry and equipment design on cannulation success is less clearly described. We report a case of difficult VV-ECMO cannulation in a 68 year old obese man with severe respiratory failure due to acute asthma exacerbation. Despite adequate dilation and standard technique, advancement of a 24 Fr drainage cannula via the right internal jugular vein was unsuccessful, with fluoroscopic evidence of cannula deformation within soft tissue. A 23 Fr cannula was subsequently inserted smoothly under otherwise identical conditions. High-resolution digital microscopy was used to quantify cannula geometry, revealing a pronounced introducer–cannula diameter mismatch and steep taper angle in the failed cannula compared with the successfully inserted device. This case suggests that device-specific factors, including both cannula geometry and nominal French size, may collectively contribute to significant mechanical resistance during VV-ECMO cannulation, particularly in obese patients. Attention to cannula design, in addition to size, may help mitigate cannulation difficulties and improve procedural safety in anatomically challenging cases.