If a soldier/athlete/firefighter made the same comments as Gwyneth about needing to work hard to maintain their physical conditioning in order to keep their job would anyone think twice about it?
Well, to my mind, the effort to maintain a high-maintenance level of physical condition is required for those jobs, but it really isn't for acting - except that Hollywood has made it so, in its promotion of unrealistic body images - normal is Hollywood fat, after all.
But being skinny doesn't make anyone a better actor.
Nor would it make anyone a better musician.