Why do consumers often think that the smaller portion is more filling when eating a properly proportioned french meal than the equivalent calories from McDonalds?
Some of it's food culture. Giant portions are normal so you don't think twice about piling your plate high. Norms (and, yes, judgement/shaming) about consumption affect patterns of same. Snacking, even heavy snacking, between meals, is common. This may be suppressed elsewhere by stronger "you eat at meal times—if not exclusively, then nearly so" norms, and snack-availability that's about what you'd expect, given those norms.
Some people think our commonly-accessible "good" food (fruits, veggies, not from specialty stores, just the main produce section of normal grocery stores) are a lot worse than what's normal in some other, healthier countries. I don't have enough experience to claim anything definitive on this, but what experience I do have does support it. If "good" food doesn't taste as good as elsewhere, or if getting something as good as others' normal produce requires special shopping and much higher prices, maybe one tends to reach for umami-bomb fat+starch garbage, which is both kinda-addictive and not very filling.
A lot of our standard cooking is tied up heavily with giant portions. We even seem to do this with imported cuisines, for whatever reason. Not-especially-good food in giant portions. Heaping plates of mediocre pasta+sauce as our image of Italian food, Mexican food with bottomless chips & salsa (and huge, cheese-slathered plates for the entrees), that kind of thing. I guess that's more of the food-culture thing.
I doubt any of these are all of the reason, and maybe none of them are correct at all.
On the other hand, foods like rice, potatoes, etc do.
[1] https://www.webmd.com/diet/features/satiety-new-diet-weapon (soft ref, appropriate grain of salt)
Food pricing, especially at chain restaurants and fast-food joints, tends to support this. It's not uncommon to pay 20% more for double the food, either because larger sizes aren't much more expensive than smaller ones, or thanks to "combo" meals. Restaurants seem to be optimizing for total sales, not margin on individual items, based on how they price—in many cases their entire menu seems to exist only to make the "combo meal" look like a good deal, but of course it may be more food than you really wanted.
To say nothing of the phenomenon of all-you-can-eat buffets...
Way, way more stores having very late or even 24/7 hours than before has probably further disrupted any norms and culture we had about when & where to eat—not just for the shoppers who have those wares available more hours of the day, but I'm thinking especially of the workers—believe it or not, young'uns, but as recently as the early 2000s almost everything in the US but certain districts of major cities were shut down and dead by a reasonable hour.
Another, possibly minor factor: I have a suspicion we have more waking hours per day, on average, than Americans did 50 years ago. You can't eat (snack) when you're sleeping, even if food's available.
I'm also very curious how much better US health would be if we could wave a magic wand and replace all sugary drinks with water. 64oz of sugar-water with a meal is a lot, but not uncommon thanks to free refills and helpful waiters always topping everyone off. Lots of people have way more than that on an average day, too. I'm sure it wouldn't fix (anywhere near) everything, but I bet that single factor is an awful lot of the cause of dietary-related illness rate differences between the US and other countries. The others have soda too, but it doesn't flow as freely and cheaply as here[0], and enormous cups/bottles of the stuff multiple times a day isn't common most places.
[0] With some exceptions—I understand Mexico, for example, consumes lots of sugary soda.