Superficial genetic differences brought about by natural selection. Same reason Asian earwax is flaky and dry (compared to wet earwax in most other areas). Or the reason that light eye/hair colors are limited to Europeans, or why Africans have coarse hair, or why people across the world have different skin colors. These traits are sometimes brought about by "purposeful" adaptation. Things like skin color are the result of changing levels of melanine in the epidermis, which allows a body to resist sunburn and skin cancer. And the lack of melanine in white people is due to the fact that the human body cannot synthesize vitamin C, which is also comparatively rare in cold climate food sources, and so Europeans had to adapt with lighter skin in order to get the vitamin C from sunlight, even if it increased their chances of skin cancer. Other traits are a mere matter of chance. Redheads are rare compared to all other hair colors because red hair is due to a recessive gene that only managed to gain a foothold in small parts of the British Isles, and so it's basically exclusive to Brits, Scots, Welsh, and Irish. I'd reckon (although I'm not certain of this; perhaps there IS an adaptive benefit to it) that tighter eyelids in Asian populations are one of those "mere chance" mutations. There's nothing particularly beneficial to them, but there's nothing detrimental either, so all it would take would be a few tens of thousands of years of sexual selection to bring about populations of people with tighter eyelids. And it's important to note there that it is, in fact, the eyelids giving off the illusion of "small eyes"; Asian eyeballs are no different in comparative size than any other human eyebalss.

this is the matter of Mendelian factor

their eyes are small or in a different shape?but I like that sorta eyes n_n

Actually, their eyes are not small.