Baby Jesus is crying now from the pain I'm feeling right now. Way to go.
The amusing thing is, I have a final tomorrow in Strength of Materials, and one thing we deal with is design factors - how much of a safety factor you should add to a material, depending on manner of loading, the material being used, and how the components are attached to one another.
If that person sits down quickly, that's more of a shock load, not static. I'd like to see that chair withstand that. But someone like that may sit down slowly, if only because all the fat on their legs would resist movement, and because the volume of air being displaced is so utterly huge that it also resists rapid movements.