You can’t really do anything in such a situation, can you? All airlines have strict regulations on the weight of the luggage that you can carry, but they don’t have any restrictions on the weight of the person or their dimensions. Doing so will obviously bring down the reputation of the airline company. I am not fat shaming anyone but the way aircrafts are designed nowadays, with minimal leg space and cramped seats that you hardly get enough room to yourself. On top of that, if someone spills onto your seat, the experience will be even worse!
If it is an empty flight with enough seats unoccupied, you can ask the flight attendant once the door has been closed if you could change your seat. Usually, if there are empty seats, you will be allowed to do so. The problem will be if there are no seats unoccupied. It won’t be considered a valid reason to upgrade you to Business Class or First Class from Economy. The airlines can neither force someone who is overweight into buying a Business Class ticket where the seats are wider.
So the only option that’s left is to adjust. You can politely request the person not to spill into your seat, but if they are too obese, there’s nothing that they can do either. You have to adjust somehow and pray that you reach your destination very soon. While the aircraft is cruising and the seat belt sign is off, you can also choose to walk across the aisle or simply stand at the rear end of the aircraft and strike a conversation with a flight attendant to kill time.