The key to getting past that is that what you are watching is the story being told to motivate the Spartan soldiers right before the Battle of Palatea a year later (which they won). So you're seeing it through the eyes, imagination and embellishments of the speaker, such as Leonidas defending against a giant Xerxes, not just a man. It's also a film adaptation of a graphic novel, and those tend to be a bit over-the-top.
Yes, I understand there’s a reason behind the style of presentation. I just don’t like it. I don’t care for “graphic novels,” either.