Part of it is that Ayu used to be so so skinny when she was younger so anything in relation looks 'big'. From interviews it sounds as though she is actually a lot healthier and that she barely ate at some points around 2000-2001 to be that small. I'd much rather have a healthier Ayu than a super skinny Ayu
For most arena tours in the last 10 years I've always thought she gains a bit of weight just before the start. I guess because it is so full on that the weight just drops off and she can eat more at the beginning to build herself up for rehearsals and touring?