Why is the HOMO-LUMO gap in N2 and CO of different sizes?
As correctly noted in the question, $\ce{CO}$ and $\ce{N2}$ are iso-electronic, so they should be comparable. But a difference remains: $\ce{CO}$ is hetero-nuclear, $\ce{N2}$ is homo-nuclear. The latter case typically means that there is better overlap between the orbitals (because they are of similar extent/energy). This translates into a greater gap between the bonding and the anti-bonding combination of the orbitals. This will definitely change the HOMO-LUMO gap. However, without performing a detailed quantum chemical calculation, it is not clear to me whether the HOMO-LUMO gap should be bigger or smaller for the homo-nuclear case, because one has to take the entire molecular orbital diagram into account.