Have lump on muscle behind knee no pain at all just happened to feel it the other day and been wondering why one leg is noticably larger then other ?

Baker Cyst: If the "bump" is directly behind the knee and not associated with pain the most likely etiology is a "baker's cyst" or popliteal cyst. These are usually benign though at times can become quite large and become bothersome due to limitation they place on bending the knee. They can, though rarely, rupture and result in swelling and pain in the lower leg mimicing a deep vein thrombois (dvt).