It's a difficult one. The FA lobby has a lot to fight back against, in terms of rejecting the mainstream (media) portrayal of attractive=thin. However, I agree that the "big is beautiful" line is just as perverse,only in reverse.
Similarly, i think they're on a losing streak trying to persuade society that being extremely overweight doesn't carry health problems when it's fairly apparent that it does.
All that said, we as a society HAVE to get over the demonisation of fat people. It's one of the last bastions of acceptable discrimination. Many people who are extremely overweight will never get within the BMI 20-25 range. There are so many complex factors at play in terms of why people get fat and stay fat that it's simply not realistic in a society with an abundance of calorific food to expect the number of overweight people to fall significantly. It's just not as simple as "If you fancy a snack, don't have a cupcake, have a carrot".
I would like to a situation where body size is just a fact. The government is just wasting money on all these obesity initiatives, and persecuting people for a bit of extra padding is hardly the sign of a civilised society.
I always think that if the aliens came down and saw how much time we spend worrying about our collective weight they'd turn to one another and say "Jeez, these guys really need to sort out their priorities"