I'm gonna say that it's alright to be big as long as you're healthy, but when it comes to a point that you are legitimately at risk for diabetes, coronary heart disease, and other weight-related health problems, then no, it is not.
I know some people are saying "If they are happy, let them live their lives," but think of it this way. If your mom was severely overweight and was at risk to die of a heart attack, then would you just tell her that it's okay to stay the same weight?