How big is it now?
One issue is that when people don't understand the needs of the fish it is not the tank size that is doing the harm, but the water quality. Most likely their nitrates are through the roof, as well as many other harmful chemicals. Any signs of HLLE?
Is there any evidence at all that a fish's organs and internal tissues will keep growing if stunted? That makes no sense biologically. Cells and tissues naturally will not exceed their space unless they are cancerous. I have heard this over and over and not once come across anything that explains it. If there is information out there I am not aware of I would be happy to read about it.
Oscars do not commonly reach 18". They can, but I have not come across hard facts of one doing it, just heard from someone who had one and things like that. I would consider 15" to be the expected size.