Happy to dig in deeper to this or other videos, but:
(
that sitrec link)
So now I'm trying to wrap my head around that small plane looking so big to me at 9 miles away as the crow flies.
The landing lights on the fronts of planes are very bright, and in a dark sky, from a distance, and especially when their beams are facing towards you in any way, they create a big glow much larger than the plane that is visible from many miles away. Even for a small plane.
At the start of the video below, the 3 lights are airplanes approximately 40, 24, and 12 miles ground distance away. A Boeing 737 MAX 8, Boeing 757, and Embraer E175LR, respectively. All under 15k ft altitude, and with their landing lights on. I filmed these approach and fly by me, in a continuous >7min long video. Only around 6min into it can you begin to tell that the last one is not just a glowing circle of light but is in fact a plane with numerous individual lights on it. (I intentionally clipped this video to not show them get close enough to see more than 1 ball of light. This video ends when the closest plane is about 6.5 miles away).
It could be helpful to try to intentionally film planes that are very far away, by using FlightRadar24 (not AR mode) to determine where air traffic control is routing planes over, and seeing if you can film planes <15k or <10k ft up, that are at least 20 miles away, and are flying generally towards you. And pointing your camera in that direction and filming enough to use the timing and angles to conclusively figure out which planes they are. Just to get a sense of what planes that far away look like. A minor caveat is not all aircraft are on all public flight trackers, but around any major airport and its common landing paths there are enough tracked planes under 15k ft to get some video of ones 20+ miles away with their landing lights on and identify the specific plane. Assuming you are somewhere with a clear line of sight, like a lake or large open field or on a hill, where trees and other objects are not blocking the view.
Source: https://www.youtube.com/watch?v=ZbLE7Sr_efw