money money money

So far as I am aware, there are eleventy billion people in the USA, about 15 million in the UK, and in New Zealand, about four hundred and ten. (Slight exaggeration.)
So how come we have to consolidate our coinage when the rest of the world continues to have one, two and five cent/pence pieces? This makes no sense to me.
About 15 years ago, we got rid of one and two cent pieces. These were our only copper coins. Now we're getting rid of five cent pieces as well. AND, all our money is getting smaller.
The new 10 cent piece is smaller, and copper coloured. The new 50 is the same size as the old 20, which is deeply, deeply disturbing. And the new 20 looks like Singaporean money, with little curvy edges.
It's all very light and small, and will not weigh down our purses and wallets so much. But still - how come a country with such a small population - and correspondingly small amounts of circulating cash - has to revise the size and weight of its coins when others do not? (It's not so long since we changed the *notes*, too.)