Very cool bun. I would guess she is wearing some sort of hair ratt or bump to get all the fullness.
I also believe there was lots of back brushing over the hair ratt. You could really blow out the hair by blowing hair upside down, using root lifts and hot ironing with a big iron (2").
This bun reminds me of Nicole's bun on the current Big Brother 16 - USA Version. Have you seen it? Super cute too.