Bigger. It’s not “somewhat”, it’s considerably. The difference between a modern and 1980s Civic is considerable. Maybe twenty percent larger.
Safer. More structural rigidity, airbags, and so oon.
More horsepower. That 1980s Civic was lucky to make 70hp. Base models now make 110.
Bigger tires. From the 10 inch wheels of the 1980s to the 15-17s of 2000. Which gives better traction and handling… but does bad things to rolling resistance.
More electronics. They eat power. Including standard AC.
And most importantly, the damn things sold like crap and had no resale value.
Finally, every five percent improvement is a major jump. In the 70s, we had a great leap forward in fuel efficency, with a correspoding complete lack of power to the car. In the 80s, power began to come back. In the late 90s, efficiency began to climb again, but it was overshadowed by the larger cars. Now, the small cars are getting attention again… and the hot rods are making 34 mpg. The modern equivalent to the GT350 or Dodge Charger Daytona; the Focus SVT, the Mini Cooper S, the Saturn Ion, the Neon … is it SVD? The Civic Si (Horrible car), the New Beetle Turbo all make 30+ MPG. (It’s a darn good time to like to drive, by the way)
Making 40 MPG isn’t impossible. Put a new-gen diesel in (Oh, yes, the improved emissions standards have done bad things to efficiency… cleaner it has to be, more carefully you have to burn it… especially for diesels), or reduce the coefficient of drag, take out all the comfort features.
For 50, you need low rolling resistance tires, removing the added structural reinforcements, and very skinny drivers. As well as a tiny car.
Above 50, it gets tricky. Those low rolling resistance tires, by the way, are lethal in the rain or snow. As in, they kill you.