Like J-F, I spent quite some years with a seat OEM as a design and certification engineer.
Several thoughts -
(a) current standards involve dynamic testing which address a measure of the actual loads expected to be experienced in a survivable mishap
(b) at the end of the day, the design mass standards are just a reasonable (but not all-encompassing) yardstick against which to measure design and performance
(c) considering (a), if the loads much exceed the relevant Standards, the occupant is likely to be in a world of hurt regardless of his/her particular mass