Review Question - QID 217360

Figure A is the radiograph of a 72-year-old female who underwent bilateral THA approximately 10 years ago. The left hip was subsequently revised secondary to the development of a pseudotumor. At the time of revision surgery the femoral trunion was found to be in good condition and the surgeon elected to proceed with a head and liner exchange with a ceramic head of a different size and a polyethylene insert. 3 weeks after revision surgery the patient sustains an atraumatic dislocation. Which of the following is the most likely cause of instability in this patient?