How to tell my stepdad I'm not Fat

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How do I tell my dad I'm not fat. This isn't a period question but it needs to be answered.

So here's the thing, I have a curvy body type (bigger hips and bum, tiny waist, bigger chest).

My weight is perfectly healthy for my lifestyle and what I eat is NOT damaging my health. I've talked to my doctors and they say at the very least I could drink more milk but that's really it.

My stepdad believes I am fat, overweight or in need of (in his words) "saving" or "in need of help".

When we go to the gym together he always goes on these long rants about how important it is to be healthy (which I understand and I find interesting) but then he'll say things that make it seem like I'm in need of this information and that he's my savior or something. It's annoying because I'm not overweight.

I've come to the conclusion that he thinks I'm fat because of what I wear. I'm a young teenager so I don't feel comfortable wearing fitted clothes because y'know (my boobs). And of course, I wouldn't want to wear those type of clothes in public , especially at the gym.

Anyways, how can I kindly and respectfully explain to him that though I don't mind going to the gym, it isn't because I'm overweight? And how can I tell him that even though it may not look it, I'm not fat?