And it ... kind of looks like him.
Granted, his voice is pretty filtered and his lines are mostly grunts and monosyllables, but Diesel actually did a pretty good job here. Unsurprisingly, The Iron Giant is his highest rated IMDB credit after Saving Private Ryan, which doesn't even count because we're pretty sure he got killed in like the first 20 minutes. Anyway, here's some footage of Diesel recording the Giant's voice.
The Iron Giant's director, Brad Bird, went on to make Pixar's Ratatouille and The Incredibles -- but sadly he didn't cast Diesel in either of them.