In reality, the 300 Spartans weren't the only Greek force fighting the Persian army; modern estimates put the Greeks' total numbers somewhere between 7,000 and 20,000, against a Persian force of 70,000 to 300,000. It was still vastly lopsided, but 300 was full of such lies. For example: Spartans didn't enter huge stabby battles wearing nothing but one red napkin over their genitals.
The film is historically inaccurate and hyper-stylized to the point of unintentional comedy, but there's actually a perfectly logical reason for these over-the-top directorial choices: The story's narrator is a Spartan soldier who wants to glorify his country.
"... and then Leonidas and Xerxes' wife totally started banging right in front of Xerxes!"
From an ancient Spartan point of view, the Persians are evil devil-worshippers who own demon harems. Persia isn't stronger than Greece because it's a richer, more unified country, but because it cheats by using dark magic. Even if actual historical records about whether or not the Persians employed bladed were-pigs are a bit spotty.
Everything about Sparta, according to the narrator, is awesome. The way it abuses its children is awesome. The way it scorns diplomacy in favor of killing people is awesome. The way it slaughters all "imperfect" babies is super awesome. Zack Snyder might be a bit insane, but he probably doesn't believe killing babies is a good thing; it's the Spartan narrator who believes that.
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Although if there was one director who would advocate killing babies ...