well. are you wearing a tshirt, or a shirt? and is it quite long? fine. and are you wearing your favourite jeans, too? (they can be every kind of size, skinny or yourboyfriend's). so. now put your shirt into your pants. and it's better if you put only a piece of it, only a side.
Cause this actually seems to be the Perfect Way to wear you tshirt on your pants! and it fits perfectly.
oh, it would be better if you would weigh around 40 kilos !