I don't think that follows. There are ways Penguin could pose a physical threat to Batman without fisticuffs, by using weaponry or henchpersons. For instance, in the animated series of the same name as this film, the Penguin, while a reasonably capable fighter himself, had two very creepy, birdlike female martial artists as his enforcers, and they were rather scary. Give Penguin a similarly intimidating sidekick -- say, Killer Croc, or maybe a version of Magpie for the bird theme -- and he doesn't have to do the fighting himself. The idea of the Penguin is that he's unassuming at first glance but quite deadly in practice because of his cunning and ruthlessness. He's not supposed to look dangerous; that's the whole point. What makes him scary is what he does.
Although it's certainly possible for a heavyset man to be an effective fighter. Watch any Sammo Hung movie.
