I could maybe stand to loose 10 lbs or so but I am usually reffered to as "slim". Even so I too have noticed the differences of seat belt lengths on diffeent aircrafts ~ to the point where I'm truly uncomfortable at the tightness of some. I've looked under the seats to seat it the belt is tangled somewhere ~ but no, they are just short.

45128 Kudos to you for having done the research and found what is needed!