There is no one-size-fits-all answer to this question. The requirements to become a model vary depending on the type of modelling one is interested in. While some modelling agencies prefer models of a certain body type, such as a slim or athletic physique because when showing off their newest fashions, designers obviously want their outfits to look as beautiful as possible. In order for that to happen, the clothes need to drape and flow, which happens naturally when they are placed on a tall, skinny frame. That is why most mannequins come in such a small size.