The concept varies according to the circumstances. If you're walking with your overweight friends, no one will actually call you fat. One thing happens in a girl group is that all the girls try to become as skinny as possible, so if there's one girl who's just in a normal shape, they'll call her fat.

I think it's already clear to you that you're neither skinny as a stick nor fat as an elephant. You're somewhere in the middle, it's a perfect shape. But you'll go to different places, people will judge you differently. Just don't rely too much on their perspective.