Would I be able to use a smart light switch here?

[dd-b]

Sorry! So easy to make things that can be read other than as intended when writing in a hurry.

The apparently-smaller wire is nearly certainly an uninsulated wire (spattered enough with paint and such that we don't reliably see the copper as different from the other insulated wires; but it's thinner because of the lack of insulation).

An uninsulated wire is, in house wiring, always a ground wire.

[danabw]

Thanks...I skimmed the sentence and read it as "...diagnostic of being a ground-in-house wire" and was thinking there was a whole new type of wire that I didn't know about.

I have those uninsulated copper ground wires in most all of my boxes in the older part of my house, and a few with aluminum ground ground wires.

At least it's not knob and tube.

[dd-b]

I'm so much happier to have knob and tube but no aluminum anywhere!!!!!

(Yeah, I could immediately see how you read it from your question, and it's not an unreasonable reading. Glad I found another way to put it that cleared things up!)

[scottgu3]

I personally think its a lot of fun to leave @danabw in the dark, but I'm just rude like that!