There is a portion size argument, but it's not the whole picture.
My plate can be piled high with sausages, bacon, tomatoes and eggs and butter fried mushrooms, but I won't be able to finish that much fat and protein.
Probably could eat twice as much Chinese takeaway, noodles, sweet and sour, prawn crackers and have pudding, and a snack two hours later.
Low carb, High fat is allows you to reduce how much you eat, without feeling hungry. Many people naturally cut out all snacks and the odd meal within a few weeks of eating this way. Hunger goes away. And even with my childhood training to finish my plate, I start thinking that I'll leave this half of the steak, or curry and have it for lunch tomorrow (yet normally, with potatoes/rice on my plate as well, I'd eat the lot).
It's hard to get your head round it after a lifetime of hearing a different story but there are some excellent books out there. And once you start reading the science, you realise calories are not the whole story, and that calories in, calories out, move more eart less is so over simplified it doesn't really work. It's a similar picture with heart disease. My fav is Gary taubes, why we get fat.