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).
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.
One of the things that struck me when I visited Paris was what felt like a lack of green spaces that you could actually enjoy. I find London much more pleasant because there are so many more parks you could go to cool off.
I enjoyed my visit to Paris, but I didn't particulalry like most of the city itself, it was clogged with traffic and was stifflingly hot. I can understand why so many Parisians try to get away from the city during the summer. Most of it certainly didn't feel particularly "romantic".
One park we went to had "keep off the grass" signs everywhere, so everyone was crammed into one strip of grass. It wasn't like the off-limit parts were particulalry special or anything, just grass with a few poorly tended flower, it just seemed to be an attitude of "look but don't touch". It was hot, dry and dusty. The London parks on the other hand are much more inviting to me, often less formal, geared towards people actually spending time in them, having a picnic/BBQ or playing games. And with the congestion charging, the traffic does seem to have improved a lot.
Of couse, not being a Parisian I might have just missed all the hidden parks, but we tried hard to find pleasant places, but struggled.