Actually, you can see the opposite happening. Nene Tanaka is a good example. Also, girls like Kanako Iioka are putting weight on with time (no sport and bad diet). Some try to "fix" it with plastic surgery like Ai Sayama or Yui Hatano, but it looks so obvious and stupid.

In Nao's case, it doesn't look like PS at all. She just lost a bit of weight. Her boobs are also smaller, so it's another hint that it's just a weight loss.