As for smaller intake tube size smaller was better for fuel mileage back when the fuel was mixed with the incoming air to keep the fuel suspended and at a higher velocity to give some ram effect. But with fuel injection the intake only has to deal with air volume and more air is better. so the larger air intake the better for MPG.
The Civic VX used a smaller diameter intake tube than the other models.
as for the smaller tube on the vx probably smaller because the vx has smaller injectors 190cc vs standard 240cc as well as smaller intake valves. still the larger tube would still allow more air in but the log jam is the small valve