All you need to know about Norbit is this:
Let me regale you with one of the better gags from the new Eddie Murphy new chef d'oeuvre. (Note that "better" is a relative term, and has nothing to do in this context with concepts like "good" or "even remotely humorous.")
Here's the bit: Rasputia (Eddie Murphy in a female fat suit generously marbled with stretch marks) draws a bubble bath, then gets into the tub, thus displacing all of the water onto the bathroom floor! Get it? Great big fat ladies displace proportionately massive volumes of water! That's sound yet simple physics—funny because it's true!
But wait, there's more: Hearing the water rushing out of the tub but not noticing that it's gone, Rasputia cocks an ear and says "Oo-ee, sound like it rainin' outside!" The humor here, you see, is more character-based, reflecting her obliviousness to the fact that she's hugely fat. It's funny because it underlines how grotesque and contemptible and beyond the pale of human sympathy or dignity she is.
Oh, and she farts a whole lot too, although that doesn't happen in the bathtub scene. Hmm, I'd go so far to say that Eddie Murphy missed a trick there. Oh well, maybe in Norbit II.