It's a health and safety issue. Depending on the airline and jurisdiction, they may have a legal duty to ensure you are not injured by being squashed up. A few years back a woman travelling with Virgin Atlantic received compensation for just such an injury, so most airlines are at least aware of the liability.
As Nean says, report it to the cabin crew. Don't be tempted to discuss it with the obese passenger, because even if they promise to try to keep from squashing you, they might fall asleep and do it anyway.
http://news.bbc.co.uk/1/hi/wales/2346319.stm