Weight, aerodynamic resistance and agility - three main criteria the car manufacturer tries to reduce on a modern day sports car. But at the same time they do something, that results in the complete opposite of that - enlarging the car to make it safer and look more dominant on the street. Even though these to points are definitely accomplished with making the car larger and rising safety regulations give them no option than making the doors and other parts thicker, I think they made the cars thicker than they would need to be.
Lets take a look at the BMW 3-series for example. The first 3-series was the E21 launched in 1975 with a length of 4,35 m (ca. 171 inches). A nice and compact car with, I admit, very little safety features but a great design. The current 3-series is code named F30 and was launched in 2012. It has a length of 4,62 m (ca. 182 inches). That's a gain of almost 30 centimeters! And if you take a look at the width the difference is even more startling. 1,61 m on the E21 compared to 1,81 m on the F30 which is a 12% gain! No surprise then, that the F30 is even longer than the first 5-series BMW from 1972.
the E21 (upper right) compared to the F30 (front) and all the other 3-series' (source: bimmerfile.com)
Another example is the VW Golf, which was first launched in 1974 and had a length of 3,70 m (146 inches) and a width of 1,61 m (63 inches). The current version, the Golf 7 grew by almost 55 centimeters (21,5 inches) in length and 19 cm (7,5 inches) in width. Especially the length is just to much of a gain considering this car is meant for the city where parking spots are not growing while the Golf obviously did. I even think the old Golf's look better, but that's just my personal opinion.
But there are some cars that could resist the temptation of getting bigger over time which is for example the Mazda "Miata" MX-5. This car actually even got smaller in length and does fulfill modern safety regulations of all countries. The first version, the "NA" is 4,5 centimeters (1,8 inches) longer than the current ND model, and only 6 centimeters (2,4 inches) narrower. And even though I know it's a convertible, I think that is the proof that cars don't need to gain in size and weight!
So dear car manufacturers, if you want to make your cars lighter and faster, you don't need to spend thousands or even millions of dollars to research new materials - just make your cars smaller and they will be lighter by themselves. And those cars will be much cheaper to produce than a full car body made of carbon fiber.