If it's hard (socially) to approach something, because people are hyper-sensitive, then it's simply hard to help those people.

In a lot of Western culture it generally seems accepted to tell people they're too skinny, but not to tell people they're too fat—obviously I don't just mean walking down a street and saying this to a stranger, but in appropriate settings.

Although I still think telling a person that you consider he or she to be either overweight or skinny is quite meaningless. If they ask you for your opinion, it makes sense to answer, but I have a really hard time believing that an over/underweight person wouldn't have noticed this already while looking in the mirror.

It's a bit like telling a cigarette smoker that smoking is unhealthy and that they should quit. The chance that they didn't know this already is practically zero, which means you're not offering any help.

What I took away from the original poster is that, it seems more appropriate in many Western societies to make available help for those who are under-weight or who smoke. It also seems more appropriate to proactively try help others too.

AFAIK too, we don't have the equivalent with smoking of people promoting that being severely overweight is okay. As for the equivalent with being under-weight, from what I see, many years ago the focus was being on skinny for the sake of being skinny as it's attractive. These days it seems to be a focus on being fit (and healthy), which generally means not being skinny, but having a more toned body, or rather, simply not excess fat.

Actually, yes. Maybe not obese, but everyone compares themselves to the general population. In a society where everyone's carrying a couple of stone more than is really healthy, noone thinks of themselves as "fat", they think they're just average.