Obesity simply comes from consuming more calories than you need. You can (and people have) loose weight by eating McDonalds if you wanted—you just have to make sure the amount of calories you get from it is less than what you actually need.
As the other commentator mentions, the only thing processed foods might bring, is trying to break our self-restraint by being delicious in certain ways.
So yes, quantity matters but you will have incredible difficulty not putting on weight from high fructose foodstuffs because your body will tell you "eat more food" to feed your muscles well before it decides to give up any fat reserves.
You'd find very little actual scientific backing for your statement, from papers that haven't been debunked[0]. That sugar itself is bad is an age old meme that needs to die. Similarly, that kids get hyperactive with sugar is also a myth[1].
Now, your other argument that you might "feel" hungrier, is something else. You have still gotten the energy you needed, it just doesn't feel as filling, perhaps.
[0] https://link.springer.com/article/10.1007/s00394-016-1229-6
[1] http://www.mortenelsoe.com/blog/sukkeroghyperaktivitet great write up if you can read Danish or translate it, or just check the sources in the bottom
Actually, the only animals I know to become obese are humans, and animals that are attached to humans.
You may say it's because they don't have an unlimited supply of food, but actually, many animals do, or used to have before we destroyed it.
Of course, they don't really forgo extra food, they use it to multiply their number instead of providing more to an individual. So it's not real restrain.