Photo GalleryStar Bodies: Too Thin?

KATE BOSWORTH

THEN: Bosworth (left, in 2003), who has always maintained she's "naturally small," once worked out five hours a day to pack on close to 20 lbs. of muscle for her surfer girl role in Blue Crush.

NOW: She recently turned heads at New York's Fashion Week – not for her style, but for her startling frail look. An insider who saw the 23-year-old actress told PEOPLE, "She's so small, it's frightening."