I've got some thinner silicone wires too, but they tend to thick yes. Still 18AWG should be more than sufficient. You won't need thick cables unless you want/need a higher ROF, and it might not make such a big difference when you have 18AWG wire. Also forgot to tell that silicone wire is waterproof (except at open ends), and that they take a lot of mechanical beating. They can also take all kinds of oils and lubricates.
"Everything will be OK in the end. If it's not OK, it's not the end."