<s>[INST] A 40-year-old man had a posterior communicating artery (PCoA) aneurysm with a 4 mm neck, which was not filling with contrast. The patient was referred for Guglielmi detachable coil (GDC) treatment. A 5 F guiding catheter was inserted into the right internal carotid artery, and a microcatheter was used to access the aneurysm. Coils were placed in the aneurysm sac, and the first coil used was a GDC-10 Soft, 2D, 4 mm \(\times\) 8 cm. What is the most likely cause of this description of selecting the initial coil size?[/INST]