The body mesh underneath doesn't change.
It's just an optical illusion: broad shoulderpads and baggy trousers, mated with a robe that looks very loose around the belly, make your character look more robust (ie. fatter) than it is. This is expecially evident on Male Elementalists.
A comparative shot. Please note that the Male Elementalist Ancient Armor was chosen because, being one of the few armors that isn't tight fitting, it's closer in perception to the Shining Blade Costume.