Interesting.

I am very overweight, and I never have an issue flying coach on transatlantic flights.

I've found most flights from US to Europe typically have more legroom, than domestic counterparts. Typically I will pay for the 1st class upgrade for domestic flights, but fly coach.

I'm 5'6 and normal weight, I did a standard flight from EU->San Francisco and on the return when checking in I bought pretty much any update that was available to give me more leg room/space. I swear I will never do another 5+ hour flight that isnt at least business class. I had back pain for a month. My feet were swollen for a whole week that my shoes didn't confortably fit, etc that I will easily lose a paying gig over doing a flight like that ever again.