[Shasown]

Obviously all the blokes commenting on this thread has some knowledge of testicles and check their own, most people would realise just as women have small differences in size of their breasts so men have two different sized nuts.

Its natural generally speaking, its only when the difference in size is very pronounced that there may be a problem. It could also be caused by Epididymitis an inflammation of the sperm storing cells. Or it may not be a problem with the large one, that could be the normal one and the smaller one could have had a period of restricted growth.