I prefer Sayu with a bit of meat on her, to be honest. I can't see her being Rika slim or anything...besides, she's not even 20 yet, in other words; she's still young.
heck, even girls like Rika can get "tummy bulges" by bending over a lot or just sitting down, that's normal. unless your tummy is SUPER toned and flat, then alright. Sayu can't even be considered "chubby"!
sure, she's gained some weight compared to before, but she's far from chubby and definitely far off from fat.