The Delta IV Heavy has a bigger fairing because it's able to lift heavier/bigger payloads, as we saw in this question. But if a payload is small and still needs a Delta IV Heavy because of its high density or large delta v requirements, can the much smaller fairing from the basic Delta IV Medium on the left be used for an additional performance boost?
In this diagram it looks like it would fit nicely... but I know that's not a valid argument.
tl;dr the company did not offer that configuration
The Delta IV User's Guide did offer two different fairings for the Heavy vehicle, one of which was 0.7 meters shorter than the other. However, neither was as short as the fairings offered for the Medium vehicle.
The longer Heavy fairing was
the baseline 5-m fairing for heritage government payloads flying on Delta IV Heavy launch vehicles.
In the image it's labeled "modified Titan IV fairing".
The image in the question appears to show the 19.1 meter composite fairing.
An older version of the guide listed an even larger 22.4 meter fairing for the Heavy but this was dropped in the later version.
