I'm a total noob and was trying to customize my plate/donut from the BlenderGuru tutorial. My goal was to add a thin gold trim around the plate. I loop cut a thin band on the surface of the plate and then assigned a gold material to it. But it didn't work how I thought it would. Why does the material show up so much wider than the faces I selected? How do I make the gold band thinner?
You experiencing this problem because of the Subdivision-Surface modifier.
When you add in a default sub-surf modifier, it just displays the modifier in the edit mode (as well as in the object mode) but won't show the change in geometry that the sub-surf modifier makes. Here is an example with respect to your model:
Notice how the geometry change when the subdivision surface is switched on.
To solve your problem just make Loop cuts with Edit Cage on.(the icon in the circle):
You are using a Subdivision Surface Modifier. From the Docs:
The Subdivision Surface Modifier is used to split the faces of a mesh into smaller faces giving a smooth appearance.
Blender doesn't know which materials you want to use and thus assigns the materials in a 50/50 ratio to the newly generated geometry.