Many factors affect your vehicle's gas mileage, performance, and lifespan. Therefore, you should be aware of what's good for your car and what's not. One lesser-known factor that can literally bog down your car is the weight it carries (particularly in the trunk).
If you've ever loaded many people into a small vehicle, then you've seen firsthand the effects of too much weight in a car. In severe cases, the body of the car may actually scrape on the tires or the ground. In any case, there are many ways in which an overloaded trunk can negatively impact your vehicle.
Any time a vehicle carries more weight than it's designed for, that puts a strain on various parts of its engine and suspension. While this won't have an immediate impact or stop your car from running, consistently overloading your trunk will shorten your vehicle's lifespan.
Additionally, carrying extra weight will lower your gas mileage. Even with the recent reduction in gas prices (as of this writing), getting the best possible mileage out of your car is still very important. The average person can spend several hundred dollars on gas each month, so it is important to save on gas whenever possible.
As far as performance, when a car is weighed down, it is negatively impacted it in a variety of ways. Vehicles are built with a specific combination of engine power and carrying capacity. When that capacity is exceeded, the capabilities of the engine (and other parts) are exceeded as well. This can manifest itself in two ways: most commonly, your car will perform significantly slower and, often times, the vehicle's steering will be affected to some degree due to weight distribution.
If you take preventive steps to ensure your vehicle performs its best and lasts for years to come, then you should consider traveling with less weight. The better you treat your car, the better it will run.