The volume expands as it heats, so it's not a given that the diameter (gauge) goes down. But the length is longer and that does increase the total resistance.

I think sag means voltage sag.

One of the more impressive things I've seen with electricity is watching a transmission line into a local neighborhood turn orange and stretch to drape itself across a pickup truck parked in the wrong spot. Took a minute to realize what I'd just watched.

You glowing orange from heat?

Yep, only thing that made sense. There was a heavy storm going through and I was driving by at night, during the day probably wouldn't have been visible at all.

In this case sag is meant literally.

https://www.atcllc.com/blog/why-do-power-lines-sag-on-hot-da...