Okay, so say I'm like a Huge elephant or whatever. And this Great White Hunter is trying to shoot me, but there's a Fine-sized insect in the way. Does the bug provide me (the elephant) with soft cover, just like a Medium-sized creature would? That seems wrong, man, but I can't find it in the rules text where it clarifies that at a certain point the size disparity between you and another creature precludes you from granting soft cover. Like, in a fight between Cthulhu and your average kaiju, would an intervening halfling grant soft cover?
I'm sure there's something really obvious I'm missing here, but this has just gotten me downright stumped!
Big Creatures and Cover: Any creature with a space larger than 5 feet (1 square) determines cover against melee attacks slightly differently than smaller creatures do. Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee attacks. Similarly, when making a melee attack against such a creature, you can pick any of the squares it occupies to determine if it has cover against you.
So the GWH just has to draw a line from himself to one of your squares that doesn't go through an occupied square, which he should usually be able to do even if there's a Medium creature in the way.
So the question becomes, if a Fine creature is between two Medium creatures, does it provide them with soft cover from each other? Nothing says otherwise, so yes, Fine creatures can provide soft cover (for instance, if there's a row of them between you and the GWH you'll have soft cover from each other). Obviously GM discretion should be used if the creature in question is a ladybug or something equally minute.
Oh! I had somehow missed that bolded part. I had come away with the impression that big creatures had an easier time gaining cover, rather than the exact opposite. That's a bit embarrassing. Still, as you say, by RAW, if you just have a straight line of bumblebees they would technically grant soft cover. Obviously, I don't think any sane GM would rule it that way (hopefully), but in this instance it happens to be important to me to know precisely what is or is not covered by existing rules text.
No, there isn't any cover provided by a fine creature to medium, let alone huge creatures. Even if one wants to argue that any semblance of cover exists, it would at most be partial cover.
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
A low obstacle (such as a wall no higher than half your height) provides cover, but only to creatures within 30 feet (6 squares) of it. The attacker can ignore the cover if he's closer to the obstacle than his target.
If a creature has cover, but more than half the creature is visible, its cover bonus is reduced to a +2 to AC and a +1 bonus on Reflex saving throws. This partial cover is subject to the GM's discretion.
Tiny, Diminutive, and Fine Creatures: Very small creatures take up less than 1 square of space. This means that more than one such creature can fit into a single square. A Tiny creature typically occupies a space only 2-1/2 feet across, so four can fit into a single square. 25 Diminutive creatures or 100 Fine creatures can fit into a single square.
Altogether, there's no reason to think that a six inch bug would block line of effect to your target, particularly if your target is huge. A six inch bug between two medium creatures wouldn't prevent or inhibit targeting in any way. Even if you want to argue partial cover applies, that's the most that would be granted, not even soft cover.
And really as Fuzzy noted, this is where a GM should exercise his or her discretion and say, "No. A dragonfly will not provide any kind of cover to the elephant standing behind it."