Why does a smaller molecule move faster than a larger molecule at the same temperature

At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get the larger molecule moving. Other factors include any charges on the molecule (positive or negative) and the nature of the material that the molecules are moving through. The molecular speed of gases at constant temperature varies inversely with the square root of the molecular mass.

As the same temperature=>KE is same
=>smaller molecule will have more velocity.