Question

why do smaller animal need more sleep than larger animal​

Answer 1

Also, the length of a particular sleep cycle is associated with the size of the animal; on average, bigger animals will have sleep cycles of longer durations than smaller animals. Sleep amount is also coupled to factors like basal metabolism, brain mass, and relative brain mass.

Answer 2

species with high metabolic rates for their size needed less non-REM sleep, not more. This suggests that they don't sleep to conserve energy. Instead, animals with high metabolic rates may sleep less because they burn more calories, so have to spend more time foraging for food.