Why do you need to be over 50kg to donate blood to the nz blood service?

the amount of blood in someone is usually according to the weight about 8 to 9% of body weight A man weighing 154 pounds (70 kilograms) would have about 5.5 quarts (5.2 liters) of blood. A woman weighing110pounds (50 kilograms) would have about 3.5 quarts (3.3 liters) of blood.

If they take say 500ml of blood, that's about over 15% of the blood in a 50kg woman, where as its less than 10% for a 70kg man