By comparison, Los Angeles is the only major US city with as much green space (the next is NYC at 27%), even though the US is 40 times bigger than the UK.
According to this, Toronto has 10 million, and honestly I'm sort of surprised it's that low... do you have a source for the "largest" claim?
https://www.td.com/document/PDF/economics/special/UrbanFores...
That's probably technically true, but a bit misleading. It's also pretty much the only city in the UK where it's difficult to get out into actual countryside. I used to live in Bristol and it certainly felt greener and more rural than London, even if it technically isn't.
Especially since they keep mentioning forests and there is nothing remotely like a "forest" in walking distance from my home in NY. Nor was there was I lived in SF, and not even in the suburbs where I grew up.
Having a forest in walking distance is a rarity.