Get yourself examined by doctor and follow his advise accordingly.
It could be normal as generally both testes are not of same size
Another possibility could be hydrocele
if it is than it may need treatment.
You have to take homeopathic
medicine for sometime after consulting homeopathic physician and your problem will be cured completely,
Temporarily you can take berberis vulgaris 30 4 tabs 4 times in day,if hydrocele is confirmed by doctor.