Is it a bit of a "how long is a piece of string" question?
Like Cervelo, we are a couple with no kids. I'm a vegetarian as well, so it actually works out cheaper.
Because we cook most things from scratch, and normally enough for 4 or 5 portions (no point in using 1/2 can of tomatoes/lentils/chickpeas etc.) We freeze portions so a meal costing around a fiver in ingredients can provide 4 dinners.
Whereas, if we were buying meat, and processed food, e.g ready meals, frozen pizzas etc, the weekly shop could double in price or more.
We've also gotten much better in reducing waste food since lock-down. Any veg that's on the way out, makes a 3ltr pot of minestrone soup, which does for the week.
I can see a weekly shop for 4, especially if two are in their twenties (assumption on my part) being a completely different animal.