yeah I've seen the "underwear" for normal hanboks..but they were no where near as poofy as the one in that pic.
According to this website: http://www.studyinkorea.go.kr/ENGLISH/A300/A300_Co.jsp
"There were various kind of underwear including pants (Darisokgot, Soksokgot, Baji, and Dansokgot), skirts (Mujigi and Daeshumsokchima), and shirts, which were shorter than outer garments in length and made of flimsy fabric in light colors. Nowadays, people wear only a few kinds of underwear such as a Sokbaji, a Sokchima, and a western-style petticoat for a flared outer skirt.
Wow..so I guess I was correct..they DO use a western-style petticoat to get the super poofy look O_O