Does being plus size necessarily mean that you’re fat?

Google "super cow." He/she is bred to have better muscles than any human bodybiulder. To some extent, people can have the genetics to be naturally loaded with muscle. Some of the best powerlifters come to mind like Eddie Hall. I'm sure he would still have been a human bowling ball if he never touched a weight. For either sex, some people can have tons of muscle or fat. In terms of clothing, "plus size" simply means the upper side of the curve. Humans vary in size very little, compared to chihuahuas and great dane dogs as an example. So when clothing companies ship a box of dresses to a store, that store is likely to need 25 size large, 22 size medium, 13 size smalls, 2 size extra smalls, and 5 size extra large.

Be a big fatso or a muscle-head or a Nordic person, 7ft3, 450lbs; body composition has no direct correlation to spacial volume.

In most cases yeah but there are some people that are really tall and/or have large muscles and they might be plus sized but not fat. 

Most likely,  but no,  no guarantees.   You know if you are fat.

To show that I am not some fat-shaming @$$, here's the cure to most obesity/excess fat. 

And how to prevent diabetes II. 

Yes it does. I mean, I'm not extremely fit. I need to lose like 15lbs myself. I dont call myself plus size, I say fat. But some women are sensitive to being called fat so they make up other words for it.