Can you gain circumference without gaining fat?

I've always been really small. Def underweight by BMI but I don't look anorexic, just small.

My measurements are 33 -21-31. My hips really child sized. My mom says it's OK because I have the waist to go with it, but still. People always say models are too thin and most of them have 4 inches more than me, so I'm wondering what they will think about me.

I almost never find adult clothes that fit, even the smallest size is usually too big. In clothes I actually do look anorexic sometimes, just because they're so loose.

Needless to say I'd love to gain some circumference to fill out my clothes, but on the other side, I don't want to look fat either. Is there anything in between?
Can you gain circumference without gaining fat?
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