I too find it extremely difficult to conduct discussion on this topic without either humour or derogatory comments and getting a few people's back's up in the process. Nevertheless, it is a very important subject and one that has affected all of us at one time or another.
As WHBM rightly points out, the standard Boeing fuselage has not changed in 50 years, while clearly, rightly or wrongly, the average human has. It is most obvious in the US, Africa and Europe, but even here in Asia, the average person is quickly getting noticeably larger. Ten years ago, at 178cm, I could see clear over the heads of a crowd in Bangkok. Now it is common to find Thais and other Asians of similar and larger size than me, and that includes their girth. Within the next Asian generation (another 10 years or so), changing diets and lifestyles will see Asia join the US, Europe and Africa in the size stakes and with it, have the same issues with airline seats.
It is time we gave this problem the attention it needs.
Bangkokeasy (BMI 28.4)
(Edited to correct BMI)