The director Zack Snyder probably emphasised the number of Spartans present over the number of other Greeks as it makes the film appear more epic having a small yet brave human force face a large overwhelming and inhuman one.
These two images compare the sizes of the opposing forces.
In 'Herodotus The Histories' Xerxes, God King of the Persians makes daily libations from Book Seven of Herodotus. "At sunrise Xerxes made libations, after which he waited until the time when the forum is wont to fill, and then began his advance." And "Xerxes meanwhile, having made libations at sunrise."(a ritual pouring of a liquid as an offering to a god or spirit or in memory of those who have died) in '300'no libations or even any offerings of any kind were made to the gods, by the Spartans or the Persians. In '300' there is very little mention of the gods made, one mention of the gods is when Leonidas's captain loses an eye Leonidas says "Dilios, I trust that "scratch" hasn't made you useless." to which he replies " Hardly, my lord, it's just an eye. The gods saw fit to grace me with a spare." Also when a storm sinks Persian ships the narrator mentions the gods. "Zeus stabs the sky with thunderbolts, and batters the Persian ships with hurricane wind." These two quotes and the presence of the Ephors. (corrupt perverted priests who worship the old gods) Who speak to Leonidas."Trust the gods, Leonidas." To which he replys. "I'd prefer you trusted your reason." The examples above and an oracle are the only involvement of Ancient Greek Religion made in the film. This is probably because the audience is more interested in the action side of the Battle Of Thermopylae rather then the religious aspect of it.
In Herodotus The Histories account of The Battle Of Thermopylae their is no mention of the actions of Queen Gorgo, wife of King Leonidas or the the events at Sparta during the Battle Of Thermopylae. In ancient Greece, women had no place in politics or debate unlike the film '300', in which Queen Gorgo argues her husbands case to send more Spartan troops to Thermopylae while he is fighting. This would not have occurred in ancient Greece as women were not considered citizens.(Although Spartan women were regarded more highly then other ancient Greek woman) Gorgo the only main character who is female probably has such a large role in the film to either draw a female audience to the film (as a female character they can relate to, or as a drama aspect of the film they can enjoy) and make it more appealing, or to make the film seem more contemporary, through the use of a lead female character.
The film '300' portrays the Spartan warriors as fearless super humans who charge into battle wearing only red capes, plumed helmets and under garments. Although 'Herodotus The Histories ' makes no, mention of the battle gear worn by the Spartans or by the Persians for that matter we can assume this is because those who would have read the Histories knew of the apparel worn by soldiers of the ancient world. However historical accounts and findings indicate that Spartan soldiers wore full regalia similar to other ancient Greek forces. Frank Miller the author of the Graphic novel '300' which influenced Zack Snyder's film of the same name commented on the body armour of the Spartans portrayed in his novel."Body Armour was a valuable asset to the real Spartan soldiers. 300 author Frank Miller commented on this alteration in an Entertainment Weekly interview, "I took those chest plates and leather skirts off of them for a reason. I wanted these guys to move and I wanted 'em to look good. ... Spartans, in full regalia, were almost indistinguishable except at a very close angle." This was an influence on the film '300' because as mentioned above they wore similar apparel to that in the graphic novel, this is another attempt to appeal to a male audience. (Or a female one, that enjoys watching scantily clad warriors in battle for 2 hours.)
A similarity between the film '300' and 'Herodotus The Histories' is the character of Ephialtes.Ephialtes is portrayed in the 'Histories as a farmer of Malis."Ephialtes, the son of Eurydemus, a man of Malis, came to him and was admitted to a conference. Stirred by the hope of receiving a rich reward at the king's hands, he had come to tell him of the pathway which led across the mountain to Thermopylae; by which disclosure he brought destruction on the band of Greeks who had there withstood the barbarians." However in the film '300' Ephialtes is portrayed as disfigured Spartan hunchback, although his role in the film is the same in that of the 'Histories' in which he reveals a goat path to Xerxes that will allow his men to outflank the Spartans, but his motive differs between the film and the historical account. In '300' Ephialtes reveals the path after his request to fight with the Spartans is denied by Leonidas, however in the ' Histories' Ephialtes simply reveals the path to Xerxes in hope of a reward. The film '300' would have had Ephialtes become a traitor to create a antagonistic character, further connecting to audience to the Spartans, and again emphasising their role as the 'Good Guys'.