If you don't believe Marissa wears a 32F, you've obviously never seen her in a tight dress. Some women just have rather large boobs. And an F cup on a broad-shouldered woman is not as dramatic as you might think, based on the breasts of size zero Hollywood types.
I wore a 36B in college (I think I was probably a 34C, but I was still listening to Vickie's nonsense then) and my breasts have, indeed, grown since then--partly because some women's breasts do, and partly due to weight gain.
I've been using the same sizing method since high school, and I have bras I wore when I was wearing a 36C and also a 38DD. They really do not fit any more. I should probably throw them out, but the ones I've kept have sentimental value. ;-)
Some people really are F cups, and it's not THAT uncommon. (I have a friend who wears an H, and another friend I don't dare speculate about.)
I think possibly somebody gave you some bad information a long time ago!