A 15 year boy presented with multiple swellings around the kne | THE WHITE ARMY

A 15 year boy presented with multiple swellings around the knee. When clinically examined he had large multiple bony swellings which were firm to hard in consistency, non mobile , irregular surfaces, multilobular with streched skin over it. X ray was done which showed much smaller bony prominence when compared to swelling clinically. What is your diagnosis? Why there is a discrepancy in size of swelling clinically and radiologically?