"How is it that something so small can inflict so much pain, swelling, and itching on a human?"
Obviously, you've never met my ex-wife. 5' tall, and even after 17+ years being divorced from her, the mention of her name causes the same symptoms for me.
BTW, I used to think the fire ants were bad. Then they warned us about bullet ants in Costa Rica. Apparently they got their name from the notion that being stung by one feels like getting shot, and is incredibly painful even 24 hours later. When we stayed at the foot of Arenal Volcano, I spotted ants getting into the room. I ended up squashing a few and blocking crevices to the outside. Thought for sure I was going to get stung in my sleep in the middle of the night.
http://en.wikipedia.org/wiki/Paraponera
These geniuses apparently decided to try out the claim that their sting is the most painful insect sting in the world:
BTW, while annoying, perhaps you can take comfort in the Schmidt Sting Pain Index, which rates the sting of fire ants surprisingly low:
http://en.wikipedia.org/wiki/Schmidt_Sting_Pain_Index
Even with that ranking, I'm glad it's too cold up here for those little bastids.