(Source: mayoclinic.com/health/hip-replacement/MY00235/DSECTION=why-its-done)

RISKS:  Unequal legs are the most common complaint by patients after surgery with over lengthening the most common problem.  Sometimes the leg may seem long immediately after surgery when they are both actually equal length.  An arthritic hip can develop contractures that make the leg behave like it is short.  When these are relieved with replacement surgery and normal motion and function are restored, the body feels that the limb is no longer than it was.  If the legs are equal, this resolves within a month or two after surgery.  If the legs are truly unequal, it will not.  A shoe lift for the short leg or a corrective operation may be needed (only in extreme cases).  True leg length inequality is caused by improper implant selection.  The femoral component may be too large and stick out of the femur further than needed.  Stiffness in the lower back from previous fusion surgery or arthritis seems to magnify the perception of leg length inequality.  (Source: news-medical.net/health/Hip-Replacement-Problems.aspx)