If I recall correctly, this has already been to the courts in the US. The answer was (I think) that the airline needs to provide seats of the right size.
When I sit down, I look for my exits and the best way to get to them (such as the counting rows method). On many occasions, I have concluded that my best route will be over the top of the seats. So-called 'Seat Surfing', due to obese people, or numbers of children.
I doubt that an accident investigation report will state, "Persons A and B were too fat to get out of their seats fast enough and so passengers Y and Z died also."
It would be impossible to prove, other than through witness statements of surviviors. So we might expect such information to emerge anecdotally in the news, rather than in reports. Either way, I expect that it won't be long.