Well, if they built a car as light as they did back in the 70s-mid 80s, you could get 50MPG from a compact car too. Probably more- the 1978 B210 MPG model was running a 2-bbl carb and primitive electronic ignition. And no catalytic converter...
But Americans want power, so a 65-HP 1.4L car with 0-60 times in the high teens/low 20s just doesn't cut it today. Plus we all seem to want our positive cup-holder to seat ratio, and you just can't LIVE without A/C, power windows, remote door locks, power seats with integrated buttwarmers, and enough sound deadening to ensure you can't hear an ambulance that's 5 feet behind you.
Add in the required airbags, compression bumpers, side impact protection, roof pillar reinforcing, you've turned even the most basic Econobox into a 1 1/2-ton behemoth.
Hence, I commute in a 1976 Datsun. Can't park a Deuce in the company parking garage anyway.