My most uncomfortable flight

This is not shaming obese people but what airlines need to do in order to properly handle the situation.

This past weekend I had a flight on United. I was in a window seat and large gentleman was in the aisle seat. I knew the flight was completely booked and someone would be in the middle with no chance to move to another open seat.

The gentlemen in the aisle needed to lift up both arm rests (aisle and middle seat) in order to fit and took over about 1/3 of the middle seat. When the person in the middle arrived, he crammed in and I tried to be accommodating giving him plenty of space. We both needed to lean the entire flight. My back and right shoulder are still sore the next morning from sitting in an awkward position for so long.

Not shaming the aisle passenger, but he was the most comfortable with his arm crossed and able to freely move them around (drink, eat a snack, etc) while the middle seat passenger and I were like sardines having to keep our arms straight or we’d elbow each other. Any time either of us tried to adjust our position, we’d disturb each other as we were more like siamese twins.

Is there anything airlines can or need to do in this situation to be able to accommodate larger passengers and not provide an even worse main cabin experience than we already are dealing with?
Is there anything the aisle passenger should have done like book 2 seats?