My initial thought was a simple "it's inside is bigger than what the exterior dimensions should allow"
That's good, but just a little cliche. It would be neat to see what other kind of geometry-bending things you could come up with. A perfectly circular room that somehow doesn't match up with pi. A hallway that seemingly passes through where another room is known to be. Seemingly impossible proportions in room dimensions. Things like that.
This particular fragment and the Site-76 part in the Containment Procedures are supposed to link this article to SCP-140, do you have any suggestion on how I could improve it?
I guess I didn't realize that. Maybe mention the connection in the article at this point. Something like "It is hypothesized that these symptoms are related to the ones associated with SCP-140." would make the connection more obvious.
isn't the addendum working to make it clear that the picture represents Quetzalcoatl being fooled to get drunk?
Yeah, but it was confusing in the main article before I read the amendment.
I'm thinking on working in a suplement to explain more about the creatures and their actions.
Yes, do this. It would make the creatures much more interesting. Right now, it's like "there's a thing that makes scary things". It's not bad, just not as interesting as if you expanded upon what sorts of things materialized.