It's perfectly normal because
2 C8H18 + 25 O2 > 16 CO2 + 18 H2O
Or, to put it another way, the reaction between petrol and oxygen produces carbon dioxide and water. In fact, it produces very large quantities of water. If the reaction inside an engine occurred at room temperature our roads would rapidly be flooded by all of the water coming from cars. However, because the reaction occurs at very high temperatures, the water is vapourised. When the exhaust pipe is still cold (i.e. just after starting the engine) some of this water condenses and runs out of the exhaust pipe. Similarly, when the weather is cold, some condensation can also occur.
Chris