Why is the neutral wire the same size as hot wire in a 3C cable?
When it is said that the current cancels each other out, it means that the neutral of a multi-wire branch circuit only carries the imbalance of the current between the two circuits of a properly wired MWBC.

So a load of 5A on one leg, and 15A on the other leg, will result in a load of 10A on the neutral.

For a typical MWBC it is extremely rare that both legs will be 100% balanced, so the neutral will almost certainly carry some current.
This is why a straight 240V circuit (which is a line-to-line circuit as opposed to a line-to-neutral) requires no neutral wire. It is a 100% balanced line-to-line circuit.

See?