2001 Ayu was 23 and naturally skinny. Asians are also smaller in general. She is 157cm, so of course she is going to look very skinny. That picture with Sawajiri is just from a weird angle.
Compare to this ViVi photoshoot for example from the same era. Short hair adds to the naturally petite look of hers.
Reading her recent interviews, I'm happy that she really cares about her health and tries to eat healthy and work out. She has admitted that she used to be unhealthy with her eating habits for example.