He is exactly the same as a regular Boo, but has been enlarged by Kamek's magic. His eyebrows are slightly different too.
Like many other bosses, Bigger Boo starts out as its regular sized species, meaning Bigger Boo starts out as a Boo (Big Boo to be exact). However he is enlarged to, as his name suggests, a bigger Boo. His actions are similar to that of a regular Boo; when Yellow Yoshi looks at him, he covers his face and becomes transparent, much like a regular Boo, except that they do not become transparent. When Yellow Yoshi turns away, he advances on them. When Yellow Yoshi is looking at him he not only covers his face, but goes transparent. It is safe to travel through Bigger Boo when he is not looking, but he can do damage when attacking from behind.
In order for Yellow Yoshi to defeat him, he must shoot eggs at the wall on which he gained from swallowing Fangs. The eggs reflect back at Bigger Boo, as no damage can be done to Bigger Boo when facing him. Whenever he gets hit, he increases in size before he almost takes up the whole room. Eventually he explodes and is defeated.
- Due to his name, it could be possible that Bigger Boo is also a Big Boo.