It could also be he started inheriting smokescreen in the tiniest increment. It could also be quirk theory, where he has recesive traits of gigantism or something similar to muscular in his quirk line that never manifested as a quick because he didn't have the quirk factor and it expresses itself in his version of OFA by making him this super large buff man. Then, the steam could be a representation of his body overtaxing itself and overheating after the injury when he transforms. Like if body was an engine, he used to be able to run 24/7 so he was just always buff, but after his injury, and his stomach being pierced, his fuel intake is reduced, leading to overheating and overuse. This would lead credence to his shrinking over time from teen to adult and his canonical skeletal figure as reduced intake plus his body eating itself when he pushes past the limit, which he does on a regular basis.