I have created a theory regarding Minya and the logic behind him. Destroy All Monsters takes place in 1999. This would mean Minya did not age or grown for 30 years. I have a theory that shortly after Godzilla vs. Hedorah, Godzilla died from toxin poisoning, leaving Minya to grow up on his own. Minya grew into the Godzilla that is in the rest of the Showa series. Somewhere along the line, 1997 would be the most practical, Minya gets a Minya, who appears in Destroy All Monsters.