I think telling a FA in private would be the best thing to do, if of course there are other seats available. If none were available the closest thing you could do would be to tell a FA, note her name and then write a letter explaining the situation to whatever carrier you're using. Who knows if you'd get anything out of it though, but it would be worth a try.

I can be a real a-hole to people in other situations like this, but growing up a fat kid and then coming into a proportional body around 20-21 in college, when I started playing rugby and exercising, makes me a little more sensitive to the whole situation and how crappy it can be. While I firmly believe people can change their habits I also don't think I could ever find the heart to call someone out about it in front of 150 other people, people you're going to be trapped in close quarters with for hours to come.

If I absolutely had to squeeze in, I would. I'd probably go uncomfortable for a few hours if there were no other seats, rather than try and get someone kicked off. However, if it were impossible, like him/her and I couldn't fit together then I wouldn't feel bad asking the person to relinquish the seat and possibly take a later flight or something.