Any team, no matter how expansively they play, needs a glorified battering ram or two.
To me it makes sense to build his size because the strengths in his game are suited to a big bloke - he runs straight and hard, is absolutely fearless and loves throwing his weight around. I think it makes more sense, and will be much easier, to change his weight than to try to change his instincts.
EDIT: I’d also echo what Wee Waa Womble says above. There’s no reason why he can’t put on weight and add subtleties to his game at the same time.