I conquered the Xeloxi Home World, it showed as Class 26 on the main Game Map, yet afterwards when I start trying to do things on the World's Map, it only had about 19 usable tiles, with one tile that I could terraform (when I have at least 4 different terraforming techs).

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.

I think you have run into a bug, as I stated I had a class 4 or 5 world when I colonized it it was missing two tiles most assuredly a bug.

Because you seemed to have a question I was trying to explain the terraforming mechanic as I have observed it so far. 

Just the normal default invasion routines. It also still shows as a Class 26 on the Game Map, and when on the Planet Map screen. I have not even researched any techs for other methods of invasion.

Also some of my examples, i.e. the two Precursor special Worlds have never been invaded so far during this game.

I have noticed on my really small Colony Worlds that the World Class seems to be how large my populations on those Worlds will grow to, a Class 2 will have 2 population, a Class 3 will have 3 population, and a Class 4 will have 4 population,etc.

So I think that World Class may be more related to population than number of tiles. But it is not exactly the same thing as population cap (or capacity).

On the other hand, my populations on my HW (Class 20), the Alien HW (Class 26),  and the Class 17 and class 12 Precursor Worlds are a lot smaller than those numbers.