Been answered before. There will be fat characters to a certain degree because hitbox is the same for all models and having fat char can be an advantage.
Better yet.
Make character models reflective of peoples builds.
For example, agility whores will have very slim character models ingame, while 45 strength (if that will be possible) crutchers will have obese characters.
Heavier armor could also weigh down high agility based builds more because their bodies technically (with their pencil-thin feeble legs and lack of muscle) will struggle with the weight more. High strength obese characters would obviously just run even slower, adding to the hilarity. (Maybe you could even see their fat seeping through the cracks of their armor
)
Note: Serious idea for the devs (minus the fat seeping through armor part)