I'm a cyclist, my legs retain water or getting bigger every time when i train hard, why is that happen?

Edema: Leg edema after exercise is usually due to gravity. Swollen legs can also be related to certain injuries (stress fracture, torn ligament). In some cases, leg swelling can be related to a serious medical condition, such as a blood clot. Elevate your legs and apply ice packs to reduce swelling. See your physician for further discussion. http://www.nlm.nih.gov/medlineplus/ency/article/003104.htm