Size wise Astra is not categorically bigger than Sol, both in absolute terms and the observed size.

The naming makes perfect sense to me, the sorting algorithm is size, both for the names and for the models themselves. Smaller things represent smaller models, and vice versa.

I would need to be a complete moron to argue with this. Humor me, however. My Latin skills end at remembering "Romani Ite Domum" and English is not my first language.

My layman reading of the names: there is one Terra, one Luna and one Sol => "Astra" is singular.

Now that I googled it, I understand that "Astra" literally means the "Stars" category. I would not have thought that they called a single star "Astrum". On the other hand, I am not sure if it means "Galaxy" in modern sense, but we're entering Roman Astronomy territory, where I am even less knowledgeable than in the language.

Sol is a star, so having a dedicated Stars category on the naming scale makes less sense. Treating Astra as Stars set without Sol, the "distance from observer" argument still kind of makes sense.

That is incorrect. "Astra" is Latin for "stars" (plural). The singular would be "astrum".

    Luna  = Moon
    Terra = Earth
    Sol   = Sun
    Astra = Galaxy