Well, I think the main reason here in Spain for thin people to get a little bit fat as they get older is the change of life that you do when you beguin to work, you have kids, etc.. you make a more sedentary life, you have less time for yourself to do sports...and you are more time sat on a chair in a office.
Anyway, I would have also size problems in the UK or US probably, unless I wanted to go to the children section. I hope not to have them in Spain, as people here is taller each time.