You mean, like this? Or this? I found better examples on my web search, but they appeared to be from personal sites, and I wasn't broadcasting that.

There are curves etc. around the belly button that will stretch and twist fabric like that, particularly if they're somewhat tight. Weight doesn't have a large bearing on it, but lighting will make it more or less obvious. If you can see it in staged pictures, you will see it in more casual settings. And it's not too surprising an artist would notice details like that.