>>538115408
The double eyelid makeup thing is to make their eyes look bigger. Anime women have impossibly large and low set eyes, basically where their cheeks should be. As a result when 3dpd cosplay they always wind up looking like they have beady little eyes and really long faces relative to the character. By drawing in false lower eyelid shapes, they can make it look like their eyes are bigger and lower than they actually are. They also extend the eye out to the sides with false eyelashes pretty much going off of the sides of their face.
This is the sort of thing where if you naturally have a small head and big eyes, you can exaggerate it by like 1% using this method and it's mostly imperceptible. If on the other hand you're a typical chinky asian with a giant moon face and tiny beady eyes, trying to give yourself anime eye proportions with makeup just makes you wind up looking like you're looking out the eye holes of a horrible skin mask.