This is Xenon, he's quite the shy Xatu, he also has a "big" secret about him that he doesn't like showing off in public, because he thinks people will laugh at him.

He was once a thin by default Xatu in the wild, but people found him and took him in as a thing to worship, and that obviously had its benefits, but now he has to hide his mass all the time with his psychic power, he only really lets it go is when he's in private, because he secretly loves it.

Maybe if he lightens upto you he will let you see him with his big belly.