Question

small intestine is a long, colied tube. why?

Answer 1

small intestine is longer because the jejunum and ileum absorbs most of the nutrients in the large intestinal parts.
to absorb more and efficiently, it is longer

Answer 2

Because thays is how human beings are made . :-D or maybe its because eventhough being much longer than large intestine, it is compactly packed within the body in much less space

Also its lumen diameter that is its thickness is thinner than large diameter so :-D
Hope it helps
also it needs to absorb much more materials through finger like projections called villi Like nutrients ,water etc so its extensively coiled. Herbivorous h ave larger small intestine than carnivorous because cellulose that is the cell wall needs to be broken dowm