The recent case with an overweight cat denied Aeroflot boarding got me curious.
Aeroflot maintains a rule that a pet is only allowed onboard if it weighs 8 kg or less. This doesn't look like an arbitrary figure, but what is it derived from?
In fact, every airline seems to impose its own requirements for the weight of pets allowed to be checked in, ranging from 5 kg (Asiana) to 10 kg (Alitalia) to hardly any known particular limit (United). (I guess this caused some serious troubles somewhere in the world with connecting flights before) However, most of the limits are observed to be around 8-9 kg. Apparently, even trains in Japan have the same sort of limit even though those never take off the ground under normal conditions.
The pet weight requirement across airlines does not seem to match their respective hand luggage weight limits either. The aforementioned Aeroflot allows up to 15 kg of luggage in the business class seat where the cat's owner was flying. Also, e.g. Air France allows up to 12-18 kg of hand luggage but only 8 kg total of a cat.
Is this 8-9 kg figure a limitation of the aircraft (hence defined this way by Airbus and Boeing), an FAA/TSA requirement, regional laws, or what else?