McNuggets are 45% meat (specifically: Chicken Breast Meat) -- at least they are in the UK, where they have to give out this information. Presumably the US recipe is at least similar.

https://www.mcdonalds.com/gb/en-gb/product/chicken-mcnuggets...

I'm sure there are many traditional Italian meatball recipes, but as one example, I had an AI convert the US measurements from Chef John's recipe, and it estimated 900g meat and 494g other ingredients, so 65% meat.

https://www.allrecipes.com/recipe/220854/chef-johns-italian-...

Of course the ingredients differ in a lot of other ways than just the percentage of meat. That's just what I looked at.