Servings are fine for candy bars, but they're almost totally meaningless if we're talking about fungible ingredients like spinach; those are going to be used in all sorts of ways, almost all of them different from whatever the relevant regulatory body had in mind. (Milk and eggs are a bit less so since they're often consumed in quanta of one egg or a glass of milk, but neither one's exactly an uncommon ingredient.)

I'm not sure there's a perfect way of comparing nutrient density under these circumstances, but volume is probably what I'd go for; you can only fit so much on a plate, so ingredients generally displace each other on a volume basis. For leafy greens in particular I might use cooked volume, since they usually cook way down.