All correct above about weight but another issue is that it affects the seat trajectory to the extent that the clearance over the empennage could become a factor for Mr Chubby. However, most two seaters have seats that diverge slightly thus reducing problem with fin. Also, for Mr Featherweight the problem could also be chute design factors such as stability or chute opening shock.
Remember that seats/chutes are designed for an envelope and generally speaking this means that they were tested and confirmed suitable for that envelope. This includes who is going to use it in terms of weight and anthropometric info and believe it or not, the height/weight data used back in the 90s was based on data gathered back in the 50s of the averages across a sample group of bods. Don't know if this has been updated yet to reflect the modern man/miss.
All that said, when I pulled the black and yellow they said I didn't go up, the aircraft got pushed down, something to do every action blah, blah.