There's not one Roman style and there were wild variations across different eras. I'll skip the archaic and early Republican stuff because it's either too rough

There's not one Roman style and there were wild variations across different eras. I'll skip the archaic and early Republican stuff because it's either too rough or derivative to be called specifically « Roman ». Late Republican busts are assumed to be very realistic. My understanding is that they're largely derived from mortuary masks made for the cult of ancestors that was prevalent at the time. It's obviously impossible to say how accurate they were without direct comparison to remains, but busts from that era show aging and « ugly » traits prominently. The Imperial era with its state-sponsored cult of personality started the trend towards idealism. Defects were smoothed out, people represented at their best and efforts were made to have them impersonate attitudes and virtues. The busts of the Pax Romana remain highly individualized and it's easy to tell a character from another, even though they always looks somewhat too good. It started getting weird with the Third Century Crisis, the Empire breaking up and the Tetrarchy. Portraiture became abstracted to the point it was more about representing the office that the person occupying it. Look up the sculpture of the Tetrarchs that's in Venice or the Colossus of Constantine, they're well known examples. Being a good fit for Christianity, that's the style that continued into the Middle Ages, and the reason why people assume that artists became terrible in « the Dark Ages ».