While doing exercise my testicle become small. Why its happening?

Dartos. There is a muscle called the dartos muscle. When it contracts, the scrotum appears smaller and wrinkled.When it relaxes the scrotum is lower and not wrinkled; When you exercise it contracts to prevent your scotum and,therefore, also your testicles from being tossed around. I hope this makes sense. It is perfectly normal.
No to worry. it's a protective mechanism of the cremasteric muscle that covers the testes. It pulls the testes closer to the body in fight or flight situations when adrenaline is pumping, as in exercise. The cremaster also raises or lowers the testes to regulate temperature for sperm production.