Not unreasonable at all, yesterday I bought DD a jacket in next, it was a blazer style with one button on the tummy, she is 7, could do up the button but would've been tight, so I tried on the next size up (9-10 [shock) and it fitted her perfectly, it worried me for a while but I came to the conclusion she is a bit larger than Next would imagine her to be, especially as the arms and body length fitted better on the bigger jacket.
I am constantly on guard with her food and it upsets me that I did this to her, you know how? Making her eat everything on her plate, giving her portions that were too large and then giving her extra stuff (homemade puddings or fruit etc, not processed junk) because I grew up in a family where we hardly had any food.
Now I've noticed this she has smaller portions is encouraged to exercise more and is never told to finish her plate (she is mroe often told 'you don't have to eat it all if you don;t want it'). DS has always been treated the way DD is now and isn't podgy, though of course at ther ages all children go through the short podgy periods and the long and lanky, but I am much more careful.
So IMO, it's sad that teenagers are like this yes, but is it really all because of the over processed food they eat? I'd imagine a lot of them are larger because of the same reasons DD is, good homecooked food but plenty of it, then we add that to lack of exercise, and processed grub and good food being so much cheaper and we have three new reasons that create a whole host of bigger sized teenagers.
Both adults and children alike need to learn the consequences and the causes without a whole host of guilt being heaped on them for doing what, in some cases, amounts to trying to be a good parent.