So you originally asked about energy per volume, but the equivalence is in mass. If by volume, premium has more energy per unit volume because of the higher density. But
@RDY4WAR 's chart shows premium as being lower density, so I guess that point isn't clear.
However, unless I'm mistaken, isn't the overwhelming majority of the energy in gasoline in the hydrogen bonds?
@RDY4WAR 's chart also shows a higher ratio of H to C in premium, which would mean more energy per unit.
The other possibility is that I'm being stupid about all of this, so feel free to dismiss this post as a semi-technical half-thought.