I was wondering if perhaps you could clarify plus size doesn't really have to mean sewing patterns or anything like that? Just being normal and healthy could put you into the plus ranges, or have your measurements so different that you feel you're odd.
One of my friends, for example, is thin and tall until you measure her butt/hips. And I count as petite until you measure my hips in some cases, and then in others I'm considered a L/M because my waist or my chest or combination of all my measurements don't fit into the "standard" assumption of clothing sizes.
So... maybe you could go into really explaining why people don't fit into "stanard" sizes and what really should be their healthy goals. Weight can be changed because muscle weighs more than fat, so you shouldn't obsess over a weight number. That said you shouldn't obsess over inches measurements being certain numbers. The healthy range could just mean being in shape and having good fitness, and eating well, which should naturally help you loose inches/weight.
I know a friend of mine slimmed down visually, but she gained like 10lbs after a few months. It was all muscle that she gained, and she had been frustrated over the scale reading 140 but she measured what she wanted- and could fit into smaller clothes.