I think they don't really have a "routine" per se ; it all depends of how they market themselves.
If you're a "small teenage girl" type of pornstar, obviously you'll try to stick to a diet who doesn't make you too plump ; but if you label yourself as thick or chubby, you won't really care about this.
Keep in mind that a lot of pornstars, male or female, can and do use some doping, though.