How much is in a handful of spinach

Judging by the nutrition information I can easily find (for example, from nutritiondata.self.com), frozen spinach is generally cooked by boiling and draining. That's pretty much what I'd have guessed; it's certainly the easy way to cook things.

Unfortunately, that means that some nutrients are lost with the discarded water. I doubt it'd be different for any other green leafy vegetable - though it's certainly much harder to find frozen mustard greens!

The nutrition information for fresh spinach is for fresh, raw spinach; if you boiled and drained it, you'd make the same sacrifice that the frozen spinach has.

As for your generalization, freezing vegetables and fruits does preserve most things pretty well - but only what's actually left in them when they get frozen! And while transporting fresh vegetables around can cost you some "freshness", it's not going to affect minerals. Some vitamins could be lost by breaking down (I'm not an expert here, but it seems possible) since they're more complex molecules, but minerals are just single elements. Those iron atoms won't fall out of the spinach on the way, and they're certainly not going to be transmuted, either!

Edit: Essentially what's been said in the comments is that the USDA nutrition facts say that "Spinach, frozen, chopped or leaf, cooked, boiled, drained" has about half as much iron as "Spinach, cooked, boiled, drained". A discrepancy, indeed, though not the same one cited in the question. My interpretation here is that the frozen spinach is simply not cooked in the same way (boiled and drained more thoroughly) as the non-frozen.

An alternative, proposed by Adisak, is that the frozen spinach nutrition means that you've taken the already-cooked, frozen spinach and boiled it again and drained away even more nutrients. This seems unlikely; frozen spinach is already cooked, so there's no reason for the nutrition information to assume that you'll boil it over again. The description (it seems to me) is referring to the cooking that took place before it was frozen.

A bunch of spinach usually weighs something like 250-500 grams. My best guess is that something like half that is the stems, which can easily be as long as the leaves. So very roughly, I'd say a bunch is 125-250 grams. Alternately, if you want to go by volume (e.g. if your store has bulk loose baby spinach), think of a bunch as something like a head of lettuce, except with the bottom third to half mostly stems.

Full-size spinach also definitely gets a lot better than 4x6 cm leaves, so you may still be using somewhat more tender baby spinach, but that of course won't cause you any problems. (The stems on baby spinach are tender enough to eat, especially when blending, so you can use everything in your bags.)

In any case, smoothies are really forgiving - not only do you not need a lot of precision, but you can just add and blend until the color and flavor look how you want!