It is all contained locally: all the characters, no matter how many sliders you slap into them, will be made from a preset. They are all basically modification of it. Think of TERA: Bob's character makes a human and chooses preset 6 and hair 31, and then moved the sliders. Your computer receives the data from the server: Bob is entering at coordinates X,Y,Z, moving in W direction. He used Preset Face #3 and Hair #31. At the same time, your computer receives the data of the sliders. Then it renders Bob in all his slidy glory. There's no meaningful difference between someone's computer sending you the data of their face than them sending you their position which is constantly changing, unlike their face/body. And all faces, textures and models are already in your computer. All the game has to do is receive the slider data which, I repeat, is no more taxing that someone's position when bunnyhopping and randomly turning. It is still not an issue.
And what Synaesthetic said.
And what Synaesthetic said.