Our prejudices--our tendency to pre-judge people--can be pernicious in real life, but you can use it to your advantage when creating characters for your fiction.
A study at the University of Texas at Dallas found that our first impression of people is based, at least in part, on their body shape, according to a report by ScienceDaily.
Generally, heavier bodies were associated with laziness and carelessness. (I know--unfair, but I'm just reporting the results!)
Lighter bodies were associated with self-confidence and enthusiasm.
Bodies considered classically masculine (broad shoulders, smaller waist) and classically feminine (pear-shaped) were associated with being quarrelsome, extraverted, and irritable.
Bodies that were more rectangular were associated with trustworthiness, shyness, dependability, and warmth.
In fiction, you can choose to use these assumptions to help define your character--or to subvert the stereotypes by, for instance, creating a self-confident, enthusiastic character who also happens to be heavy, or a lazy, careless one who happens to be skinny.
I prefer the latter, not only because it may be a tiny step toward undoing this kind of pre-judgment but also because readers like being surprised and if they subscribe to the current stereotypes, this kind of reversal will surprise them.
