Leather Cuirass Lorica Musculata, I used to think no way but

[Tarbicus]

Who wants to stand in front of a sculptor for hours all day in heavy bronze armor, when you don't really need the protection? Perhaps on the statue of Antoninus, that is his faux-armor beside him and not his subarmalis?

Do you need the Emperor's body for the portrait, and not just his head? Looking at the heroic statues of Augustus, he does tend to look a bit 'beefed up'. But that said, your last comment is viable.