You might be amazed at how people devour roast potatoes. About 15 years ago, when my kids were still here, we used to do a Xmas lunch. I distinctly remember peeling 15 kgs of potatoes for about 25 people and my wife hiding a portion so I could at least eat some.Wait--do you mean you cook 2kg of potatoes at Christmas, but are there no leftovers? They don't actually eat all of them, right?
We'd do a huge roast turkey, Brussels sprouts, carrots, roasted beetroot, and a pile of desserts.. but the potatoes were sacred!