A reduction of 20% is most certainly significant.

What proportion of them are in prison for possession of small/personal amounts?

And is imprisoning people who are smuggling/distributing hard and potentially very dangerous drugs really that controversial?

I have no idea, I don't live there. I'm responding to the assertion that 1/5 is not significant.

no one asserted 1/5 is not significant. you misread this.

i was just stating the fact that the initial number posted was wrong/incomplete.