[neaanopri]

One of the reasons liquid methane works well as a rocket propellant is that rockets need to carry their own oxidizer with them. Because methane is a smaller molecule, less oxygen is required to oxidize a given amount of methane.

With an airplane, which gets its oxidizer from the atmosphere, methane's lower oxidizer requirement is basically irrelevant.

[s0rce]

The amount of oxidizer on a molar basis shouldn't depend on the size of the molecule, just the number of C and H atoms that need to be oxidized to carbon dioxide and water, respectively. Some molecules (ex. ethanol) also have some oxygen already in them which reduces oxidizer needed but also reduces the energy density of the fuel (its basically partially burned already).