Is there a difference in iPhone body?

This may sound like a really dumb question, but I have reason for my suspicion.
My mom had a black 16gig iPhone 4. The family upgraded phones and in it I got a white 8gig iPhone 4. My mom gave me one of her cases and it fits perfectly, but the silencer switch and volume buttons are a little off.
When my brother and I went to Best Buy, some of the cases said specifically for iPhone 4 16gig and 32gig. Others said iPhone 4.
Is there a difference at all?

Are you sure they are iPhone 4s? Reason being, there's also the iPhone 4S which looks almost exactly like the iPhone 4 except that as you described the headphone port, silent switch & volume buttons are placed differently.
That's the difference between the 4 & 4S, externally anyway. There's no difference between capacities of iPhones. Like an 8GB being different from the 16 or 32GB.