Wait, there are i-Size cars?
Yes, car manufacturers and child restraint manufacturers are now working together to ensure that child restraints will fit into all approved cars. This is achieved by the vehicle manufacturers making the interior of the vehicle to approved specifications.
For the child restraint (car seat) to be approved for iSize, its size and shape must fit within a set sized 'envelope' and the support leg must be long enough to touch the floor of all approved cars, even those cars with under foot storage (so support legs will be longer).