I thought it was clear that this was a stunt suit and a stunt actor, as the mouth doesn't resemble Keaton and apparently Keaton himself only flew in today/late yesterday.
You can see from how the midriff area flexes/crumples with the movement that this must be a stunt suit. The upper torso/chest area appears to be more fixed for aesthetic purposes I guess, therefore making the chest appear larger and casting a larger shadow across the lower half. I'm thinking this will be for action and wide shots, with Keaton himself wearing a more refined/streamlined suit for the closer shots.
I may be wrong, of course!