If I tried on one of my bra cups as a hat I would reject it as being too large.
Um, yeah. I have.

So maybe not a full adult head, but bra size bigger than hat size and that's plenty big enough to be unwieldy.

"... And why would I have wished for a ten inch pianist?"

have you tried putting your hats on them?

Ok, that sounds kinky.

You've seen them, V, you know they'd fall off.
And Foot3 :notworthy I'm honoured.

I was reminded of an old Designing Women episode...can't find a vid of just this part but here's the transcript:

The ladies discuss Ursula taking their men's attention......
MARY JO: Well, I don't know if we could compete with her even if we wanted to.
SUZANNE: Speak for yourself.
MARY JO: Suzanne, I don't want to hurt your feelings, but I saw her bra hanging on the towel rack in the bathroom.
SUZANNE: So?
MARY JO: So you are not top dog anymore.
SUZANNE: How do you know?
MARY JO: Because -- I can get my whole head in one side of hers.
CHARLENE: Mary Jo!! You put her bra on your head?
MARY JO: I couldn't help it!! You know how that fascinates me. Suzanne's cup fits me like a beanie, but Ursula's covered my whole face.
SUZANNE: Well, Mary Jo. This just helps to point out the main problem that you people have that I've been talking about. You are not competitive! I mean, here you have this gorgeous girl who gets up at 5:30, cooks your boyfriend's breakfast, then goes jogging with him. And what do you do? You're in the bathroom trying her brazier on your head!! I mean, you're as impressed with her as he is!!

And after that, they sat down at their Memotech MTX512 and created Meshach Taylor.