Since it was a Class 26 World with only 19 Tiles currently available, I thought that a possible explanation might be that there were another 7 tiles that could be terraformed into usable tiles. Which is why I mentioned that I had 4 different ways to terraform tiles, as I have done and used already on other planets that I owned. There are still one or two more ways to terraform that I have not yet researched (definitely via the gene resequencer Tech that I have done in previous games of the 0.4 Alpha for GC IV but not yet researched in this game).
I also wonder just how many tiles did the AI Race have available when it was their Home World, could they have had 26 tiles in use and 7 of them went away when I took control of the World.
By the way, at least several months later, it is still showing as a Class 26 World whenever I go to that World to do things, like upgrade the tech on the various types of Districts on that World which is now a Core World for me.
Plus in the same Game, at my current turn, there is a Core World that I also invaded and captured from the same Alien Race that was "Once the capital of the Precursor Civilization". It showed on the main Map as a Class 29 World, and I think that it had 29 tiles that were developed, i.e. had Districts on them, when I captured it, and I was able to terraform an additional tile after I captured it. So it currently has 30 tiles being used (by me) and still shows as a Class 29 World on the Game Map and on the planet map screen as well.
The captured HW and the Class 29 World are in a different Sector than my Home/starting Sector.
As additional examples of the problems of what does World Class mean from the same game? :
My Home World (HW) shows as a Class 20 but it currently has 25 tiles in use, I think that I terraformed most of the additional 5 tiles and/or I probably used once the special item that you can discover during surveying anomalies that adds 2 tiles and 1 Food production when you use it on a World.
A Class 17 World that was "A Precursor world built from a crystalized Promethium" that I got early in this game, it may have had 17 tiles when I got it but it currently has 22 tiles being used by me after they were terraformed by me. As well as getting one of the add 2 tiles and 1 Food special, used on it.
A Class 12 World that was "A Precursor world that had been completely converted to be a refinery" that I also found in the same Star system only a couple moves away from the Class 17 World (see the previous example) that currently has 14 tiles in use with 3 more tiles currently in the production que to be terraformed (by my currently available Techs). I think that I may have used another of the add 2 tiles and 1 Food specials on this World as well.
Both of those Precursor Worlds have been my Core Worlds for 4 or 5 years now in that game.
So I and others can see that there is a disconnect between World Class and the actual number of tiles being used and/or available to be terraformed. Apparently the World Class figure never gets changed/updated in the current Alpha for GC IV, to reflect changes to that world. I think that in previous versions of Galactic Civilizations the World Class did get updated as changes were made to that World. (PS I have played Galactic Civilizations from the days when it was the original game before any expansions or even the addons in the original game, about 20 years ago).
I suspect that if I looked at every Core World that I own, that it's World Class has never changed from what it was when I got it even after several tiles were terraformed by me over the course of this game and previous games of the 0.4 Alpha of GC IV.