Then I realized…..
He totally thought of that!!
In his letter giving instructions about the performance of Don Juan Triumphant he says, “Our Don Juan must lose some weight, it’s not healthy in a man of Piangi’s age….”
I always took think to be another instance of comic relief at Piangi’s expense, but no! He needs Piangi to lose some weight so that he can take his place during the performance without being discovered!!
Maybe everyone else has figured this out already, but I feel pretty clever right now. :)