The length of small intestine depends upon the food the animals eat. If it a herbivorous animal , then it it will require a longer intestine because the cellulose content in the food takes a long time 2 be digested ....On the other hand if it is a carnivore then it will have a shorter length of small intestine as meat is easier 2 digest n it will take less time as compared to that of cellulose....
The length depends upon the mode of nutrition.
The hervivores animals have a large intestine where as the carnivores has a small intestine.