As a rule, when you contract and relax a group of muscle repeatedly, you increase the blood circulation.Groin is an area where blood circulation is lowest. Groin area are include, base of the penis, muscles surrounding urethra of penis, Scrotum, containing testicles, perineum(small muscle between anus and the base of scrotum and the whole of anus including tail bone muscles.One can all muscles all at a time or one by one.A brief method can be as under. All muscles at a time. you take an abdominal breath.It means you abdomen swells while inhaling not chest. while exhaling you abdomen goes down toward your spinal colum. During exhalation contract all groin muscles and release all muscles when you inhale.Do it a set of 10 rounds. gradually one increase as per their capacity. You may also do one by one. Inhale an abdominal breath and exhale contracting base of the penis. Then take another muscle. You can do all muscles one by one. By this exercise you are moving sexual energy form one end to the other end of the groin. This is a part of Chinese technique.It is similar to Bandhas in Hath yoga. Google Sat kriya and Microcosmic orbit.To understand the concept.
It could be hormones or some change of diet. I don't think it has anything to do with masturbation. My testicles got smaller as I got older too.