What? The fact that some people eat enormous amounts of fast food without getting fat hardly means that fast food cannot be making anyone fat.
Additionally, you seem to be assuming that skinny = healthy, and fat = unhealthy.
Finally, I'm fairly certain that studies have shown that as western fast food restaurants expand internationally into new cultures, incidences of obesity, diabetes, and heart disease all rise in those cultures.
Ceteris paribus, skinny tends to be healthier than fat, with longer life and better quality of life. One study cited here even indicates skinny people don't even have to be that healthy otherwise to outlive fat people. It's a simple "assumption" that requires more debunking than you have given here.
Studies have also shown that obesity is well linked to disposable income. Is it any surprise that restaurants also expand as disposable income increases?
For breakfast get an egg mcmuffin, no hashbrown and grab a coffee or orange juice.
If you just stick to those two meals when you have to have fast food it does basically no harm.
Fast food gives you the rope to hang yourself with, you don't have to do use it.
I'm not saying that eating lots of fast food isn't correlated with being fat. However, I would say that since it's quite possible to be skinny and eat enormous amounts of fast food, fast food cannot be making people fat. I would expect that the same character traits which lead someone to eat lots of fast food would lead to being fat for people predisposed to that body type.