I had the same thing happening to me flying back home in May (Lufthansa, Charlotte, NC to Munich, Germany)
I arrived at the very last moment at the gate (and had to get my boarding pass there) because of a delayed flight out of Orlando, and I was "lucky" to get the last seat (WTH??? I mean, I had reservations for that flight! I paid that flight. Well, the company paid, so they shouldn't treat me like someone who is lucky enough to win a lottery to get a seat on that plane!)
Since I had the very last seat, and I got on the plane as the very last person, it was impossible to switch seats.
I was in the middle part, first row (configuration 2-3-2 on first row), the middle seat. At my left side was a lady, at m right was a man... a Russian giant. I am not small, but that guy was twice as tall as I am, and at least three times as big. When he sat down, his leg crushed mine, and his arm was all the way in front of my chest. And did he smell nice... NOT! Think he had never heard of deodorant...

Lucky me, since at the first row, you can't put the arm rests up, since the trays are inside those. And even more lucky me, that somehow, before departure, the lady at my left upgraded to business and moved, so I could move too.
I can't imagine spending 9 hours completely crushed by that man. Nine hours having to hear him snoring (because he snored even when he was just breathing and awake!) right in my ear. I may not even think about all the complications I could have had because of his leg crushing mine (I have had a thrombosis, so long flights are already dangerous for me, my legs being "crushed" is dangerous, and both together is the recipe to end up in the hospital (at best) or on the cemetary (at worst) )
I know larger people have to be able to fly too, but something has to change. I mean, it's not that the larger people are the exception anymore. Maybe it's time for the airlines to have "special" seats in economy class that are a little wider than the regular ones, just like United airlines has the ecenomy plus with more leg room (ideal for people with long legs like me + for people with health problems like me). I guess those seats are more expensives than the regular economy ones (not sure, company paid) but I guess in case they provide wider seats for the larger people, they will not be as expensive as having to buy 2 tickets/ seats.
PS: for those that can't close the buckle, you can always ask for "extenders". That way, you too, will be safe in case of turbulence etc.