Hmm... I remember a long time ago, I (jestingly) came up with a theory on why girls with large breasts are stupid. Here's a rough translation:
Well, breasts are mostly fat (with some glands and the like). Notably, the brain is mostly fat as well (with some nerve cells and the like). I hypothesize that in order to have the amount of fat needed for larger breasts, the fat must be taken from somewhere else in the body. The fat of choice is that in the brain. Since this detracts from the cushioning that the fat provides, nerve cells in the brain are killed at a much increased rate as breasts develop. Therefore, girls with large breasts must have smaller (and inferior) brains.
Note, this is all a joke. My oldest sister is a D cup, and is one of the smartest and most wonderful people I've ever known.