This makes sense when you consider most green vegetables like broccoli are 90% water. A meat like chicken, after cooking is only about 60% water.
So if you eat 400g of broccoli vs 400g of chicken, the one that will make you feel more full is obviously the protein.
What’s more likely, is due to marketing we’ve associated anything that is green in color with being “real, nutritious food” and have convinced ourselves that is what makes us feel full.
It’s like people who claim they “feel better” after eating a salad. The average iceberg lettuce has near zero nutritional value and is 96%+ water.
The only way you’d feel better after eating one is if you were dehydrated.
-- *edit - people seem to be misunderstanding. I'm not saying fast food is healthy. I'm simply refuting the idea that green vegetables are the key to feeling full. If you wanted to feel satiated, green vegetables would be one of the most inefficient ways to do so--due to their 90%+ water content.