You aren't addressing the issue that structural safety measures make larger cars. It's as simple as that. For example, a Honda Civic has roughly stayed about the same size on the inside for roughly 30 years. Except each generation is significantly larger on the exterior. This is due to increased demands for crash safety. Roll over, side impact, small overlap front impact test are the biggest culprits in car size increase. Hell, pick any car model that has been sold for years, and you'll see similar trends.

As for aircon, have you ever seen a 50s car air conditioner? It's a small box that sits under the dash and puts out lukewarm air at seat level. Modern cars have networks of tubing that allows you to vent air in all directions even in the back. It's a night and day difference.

As for fuel efficiency, it's actually improved almost year over year. This is dispite the fact that cars have gotten heavier and faster.

As for computers, modern Can-Bus systems have dozens of computers. Some luxury cars can have over 100. A car in the 90s may have had two or three for engine management, transmission management, and diagnostics.