It might be a detail, but the two large parks (Boulogne and Vincennes) are within the city limits.
Also I don't know London very well, but from Google Maps I see almost no greenery in the center, while Paris' parks are maybe smaller but much more evenly distributed throughout the city.
Are you joking?
Regent's, Hyde, Kensington, Green, St James, Battersea, Burgess, Kennington, Coram's, Russel Square, Lincoln's Inn...
Look at all the green areas on the map.
Paris: https://www.google.fr/maps/@48.85615,2.3306852,14z / https://www.google.fr/maps/@48.862458,2.3322563,12.75z
London: https://www.google.com/maps/@51.5160429,-0.1466301,14z / https://www.google.fr/maps/@51.5184651,-0.1436204,12.75z
The parks in London just look larger and fewer, not very evenly distributed, to me.
http://www.worldcitiescultureforum.com/data/of-public-green-...
It's one of the things I really appreciate when I go to London, there's always a park within walking distance or a short bus ride/tube trip.
Probably not that much, you can pick it up through google maps: Paris has large parks & forests outside the city (boulogne, vincennes, meudon, …) but only a few very small parks within city limits (the largest green space is the Père-Lachaise cemetery), nothing like Regent's or Hyde's.
It's really flagrant using the satellite view at about the same zoom level (using the scale), it's as if london had no major park closer to city center than Hampstead (though both boulogne and vincennes are quite a bit larger than Hampstead, they're on the same scale as Richmond Park).