Most people will want a heated vest to fit slightly more snugly than a normal vest. The main reason is because if the vest is loose, you might not be able to feel the heated elements. This applies for just about any heated garment. Especially when running the heat on the lowest levels. I’ve personally found that most of my heated gear needs to be run on medium at a minimum for me to feel the effects. Try to keep the layers under the vest to a minimum because that can also affect how much heat makes it to your body.