I have some relevant experience here. They put some shit in fast food that suppresses your ability to feel full. I'd swear to it in court. Let me explain.

Working all the time and running a business, I ate fast food pretty much every day for years. During that time a "healthier meal" was eating at a real restaurant and getting like, fajitas. I had a voracious appetite. My standard McDonalds order was a large Big Mac combo plus two double cheeseburgers plus 2 McChickens. And I would eat all that garbage and still feel like I could eat 2 more doubles afterward. Everywhere was like that. Then I moved and had a nice kitchen, and was going through some rough spots and started cooking again to save money. Except the portions were different. I could cook an 8oz steak and put a couple spoonfuls of potato salad on a slice of bread and be full. Literally 1/4 or less of the portion, and not only was it good I was stuffed afterward.

After eating like that for ~6 months the boss ordered delivery at the business one day and I got my usual McD's order and it was disgusting. The taste, the texture, everything about it was wrong. I tried out other places and it was the same everywhere except for Wendy's and like, rice-heavy Taco Bell items. I think they use real food in their shit and not whatever is going into other fast food. But to be able to eat that much and still be hungry makes no fucking physical sense.