I am Devilman, the Devil!(1991)

As Akira Fudo wakes up, he finds himself in a "chibi" form, which he thinks doesn't fit a hero such as himself. He is not the only one that has changed. Together with Ryo and Miki, they set out on a journey to restore their proper appearances and find out what caused this change. During their adventure they find several other characters from the world of Go Nagai, such as Mazinger Z, Dr. Hell, Baron Ashura, and others.