Mark Antony gradually succumbs to the ostentatious luxury of Egypt and the seductive allure of its beautiful queen, Cleopatra. In doing so, he earns the contempt of many of his fellow Romans. Noble Roman warriors are supposed to be hard, manly, and spartan. They're not supposed to throw themselves into lives of unabashed indulgence and allow themselves to be ruled by women, least of all foreign women. Yet that is precisely what Mark Antony does, and in the process becomes somehow less Roman as a consequence.
Octavius, on the other hand, likes to portray himself as an unstinting champion of Roman values. He manipulates traditional Roman xenophobia and misogyny to paint an unflattering portrait of Mark Antony as a kept man who's sold his soul for a life of ease and luxury. Octavius doesn't really change throughout the course of the play, and this is undoubtedly one of his great strengths. Unlike Mark Antony, he hasn't been distracted from his single-minded goal of consolidating his power at Rome. He can therefore turn all his attention to defeating his former colleague and his Egyptian queen.