- Purchase of a second seat costs 75% more.
- Extra fee applies only if the flight is full.
Personally, I don't think there should be a single cut-off after which you pay twice. Instead, a portion of the calculation of each passenger's fare might be based on the passenger's weight. Ideally, a tax on junk food -- soft drinks, candy -- might compensate airlines for the cost of transporting fat passengers.
The new policy appears to posted at the website of Air France under the heading "Passagers à forte corpulence" (English translation via Google here).