Can I connect 12 gauge wire to 18 gauge wire for an outdoor light post and plug?
The fixture is a listed device and manufacturers don’t have the same limitations for the wiring that electrical wiring in your home has. YES you can connect the 12 AWG wire to your light pole fixture. This is exactly the same as almost every light fixtures in your home mfg’s regularly use smaller wires because the load is limited.
 Note they have a maximum wattage listed for the light(s) when used according to the manufacturers instruction it is safe. So yes this is normal for a fixture.