Doesn't this work both ways? If you focus on MPG then lighter, smaller variants of the same model will win out over bigger, heavier ones.
But... that car would be a dud today. It's too simple and lacks features (like power seats).
Hence, smaller cars have better MPG. Smaller cars also tend to be lighter.
For in-town weight matters greatly (both because you essentially throw away energy every time you break, and because rolling-resistance matters more at low speed).
For highway, up until 2008 the maximum speed tested was 55, which disadvantages heavy-vehicles, as rolling resistance is a higher component of energy used at lower speeds (as a simple approximation, rolling-resistance is linear, and air-resistance is quadratic).
A car that is 50% heavier does 5 times the damage.