It is not portion size which matters
It is what's innit and what do you do besides stuffin' your face.
I can stuff myself with triple Balkan (Bulgarian, Serbian, Macedonian - they are all about the same) grill portions up to twice a day (that is a plate size pork chop each) and still keep my weight close to its healthy numbers. This is despite not overdoing the exercise and having a job which requires me to spend most of my day staring at code on the monitor in front of me.
Junk food (as depicted), however, in the days when I was still eating it meant 10kg+ on top of it before I bat an eye lid. My personal suspicion is the bread. The bread today has nothing to do with what we ate as kids - it is full of "interesting" additives including some really nasty brominated sh*t which allow it to stay nice and rubbery for a week instead of becoming hard in 2 days as bread should.
So it is not the calories, calories in that range can be burned off and are burned off. In fact, you can burn off significantly more surplus calories without noticing them. It is something else.