The trend to smaller is already happening, but don't expect a downsizing of current models. In the 70s the shift happened with introduction of new models. What will happen is a line of cars will come in under the Civic (Jazz/Fit) and basically slot in where the Civic did a decade ago. The Accord won't need to change, but may need to add its wagon version again, as SUV drivers downsize to family sedans in the Accord size. The RDX comes just in time, we can only hope its getting gas mileage in the 25-30 mpg range (or of course better).
Also, different from the 70s, kids these days are just plain bigger and taller (and we know fatter). If you have say boys in the 8-10 years old and are looking for a car to get you through 5-7 years, it very well could be that those kids will need serious leg room more than the Civic offers...
Carmakers won't be quick to provide smaller, but will be quicker to up mileage on current offerings and call and end the outrageous power wars of the 90s.