The increased efficiency of your heart and lungs to get the oxygen into your body and pump it round to the muscles that needs it. A fit person can run at a steady rate for many miles and barely get out of breath. An unfit person of the same weight be puffing and parting after a few hundred yards. This fitness is achieved without particularly increasing the size of heart and lungs but just by improving their efficiency. Fit people are as it were turbo charged getting more power without increasing the size of their engine.