While there can be a small difference in the size of the two testicles, if one is truly twice as large as the other it could signify a problem. Problems such as lack of blood flow or too much blood flow in the smaller testicle can cause it to stop growing, an infection, or an abnormal growth in the larger testicle. A health care provider can measure the size of the testicles and check for some of these problems. If he/she finds a true size discrepancy, an ultrasound of the testicle can help to find out what’s causing it.