Increasing the octane actually reduces the energy content of the fuel.

I thought it just made it less susceptible to detonation.

That's obviously the main feature and why you would want higher octane fuel.
However, it has the side effect of reducing the energy content of the fuel.

Take LPG, for example. This has a much higher octane rating and so can be
used in petrol engines very successfully without detonation. However, it has
a much lower energy content, which is why it gets much lower mpg than 95RON.

There are methods to restore the energy content of the fuel whilst
maintaining a high octane, but it won't happen by default.

Christian.