Because wrist size changes with weight change, does that mean it isn't an accurate indicator of body frame size?

It's Ok: Wrist size can be used as an indicator of body frame size. It will be inaccurate at the extremes of weight though. It is not perfect but it will give you an idea. https://medlineplus.gov/ency/imagepages/17182.htm

Doesn't change much: Dr. Pool's reply was correct. Wrist size is used for this purpose because it does NOT change much with weight change, except for very slim and very obese persons. For most people, it's a fairly accurate indicator regardless of body weight. Not perfect, but a good general guide.