Wife and I talk ALL the time about anything and everything, be it the weather, how weather works, of free will exists, the kids, if kids exists, you name it...
We're both introverts, so we're totally comfortable just sitting next to each other reading different books, or cuddling on a cold winter night. Sometimes we talk about random stuff, but quite often we're exhausted from dealing with other people but still want that proximity.
If you went outside and the weather was pleasant you'd never mention it to your wife? Never say anything like "have you been outside? It's so nice today!"
Genuine appreciation for the weather enough to declare it to those around you isn't small talk. Small talk is generic filler dialogue you do as a formality.
If you're being that reductive about the definition of small talk then I don't think small talk exists between couples who have known each other for a long time because you're just regular talking.