Why do you care so much about a few inches' difference in the width of your car? If newer cars had inferior gas mileage due to their size I would understand the concern, but they don't... not even close.
Do you have an extremely narrow garage? Do you regularly drive down very tight off-road trails? Are you one of those people that's really bad at and/or nervous about parking between other cars?
They do get inferior fuel economy. If we look at Honda, for example... my midsize car(Subaru Legacy), which used to be the same size as a Honda Accord, but now is more similar in size to a Fit(even the Civic is 3 inches wider!):
A manual trans Accord 1.5 gets 30 combined, while a manual trans 1.5 Civic get 33.
I didn't engineer the car, so I can't say for certain, but size and weight would seem to matter here. But more to my point, is that there were larger cars for people to buy before. But they bought the smaller ones, complained they were too small, so companies made them fat. This is reflected in car reviews all the time, when people complain about things like back seat room in a Ford Fiesta. It's SUPPOSED to be a small car. You don't buy a Fiesta to take a family of 4 on a cross country journey. You buy it to commute to work in.
And again, in 1989, the CRX HF was getting 44mpg combined.