Why do WeakMap and WeakSet omit iteration and size?

Why do WeakMap and WeakSet omit iteration and size?

1. Enumeration would expose garbage-collection-dependent liveness and make ordinary program results nondeterministic.
2. Weak collections store entries in insertion order but hide that order only for speed.
3. JavaScript objects cannot be iterated by any built-in collection.
4. Calling size would force every value to become a primitive.