As with all engineering, the answer is not as simple as it looks. Just designing for heavier occupants may have other consequences. This is a subject I discussed in my years in airline seat design and scaling the seat to suit really heavy people can make it unsuitable for really light people. I noticed an example of this today in a UK AAIB report on an Extra crash in 2008 in which it was thought that the heavier occupant was protected from some injury by his seat cracking (hence absorbing energy) while the lighter occupant's seat did not crack and she suffered worse spinal injuries.