Computer power supplies usually have higher efficiency on 230V than on 115V. Why?
As the law \$ P = U * I\$ , to acheive the same power at lower voltage, you need to increase the current.

In resistive components, like wires, pcb traces, transformer wire (green), losses increase to the square of the current, as \$P(loss)  = R * I ^ 2\$.

In switching components and other diodes/rectifiers, (Green) the losses equal to \$ P(loss) = V(bandgap) * I \$. 
V is bound to the component regardless of the voltage input, like ~1V for a rectifier. 

Eddy currents losses (Red) will also increase in any core as the current (and thus electromagnetic field) increases.



Losses related to capacitor leakage are negligible.