I'm 5'9" and 155 pounds. I normally wear a 36/38 C. I think it's a natural size for me.
One Halloween, I went as a Hooter's girl. My friend didn't think I met the definition of hooters so she convinced me to wear her extra pair of C forms over mine. I did, but still didn't feel overly large. I also have a ginormous pair of forms from China. They're about a 38FF.
One time I was on a site and they were selling a 38N bra. I had to buy it just to see how big it was. There was no way that I could afford that much silicone, so I padded it with all the socks that I had clean, and I almost ran out of socks. It looked humongous, and was so big, that I didn't have any clothes that would fit over it.