But many people manage to stay slim despite their genetics due to ridiculous amounts of hard work. So I still find it insensitive to say "lucky" regardless.
What if someone was thin because they lost a lot of weight due to cancer? Wouldn't "you're so lucky" be horribly rude at that point?
I think if a comment can be misconstrued, it's best not to say it.
Being an etiquette board, I don't know why we're so willing to wave this off as, "Well, that's not how it was meant despite the fact that literally, that's exactly what it means" and label the offended people as "oversensitive" and "special snowflakey". I know that I, at least, have learned from this board many things that I thought were innocuous could actually be hurtful or rude to other people. As a result, I now make a conscious effort not to do those things.