I think that Ayumi only recently began to work out (around the 'fairyland' time through her last tour). However, I think if she's still working out, she's lightened out the exercises. It could be for her own enjoyment or to keep up with the younger group of girls out there who do dance a lot more.

Anyways, Asian countries aren't as healthy as they were before with the introduction of more fast food and snacks into the area (additionally, skinny doesn't necessarily mean healthy). However, I think perhaps high metabolism is more common (my mother at that age didn't weigh more than 90 lbs and even now she's only 110 lbs, whereas I'm 102 lbs)