Oh wow, 20,000.... I thought you meant, like, 25. 
The goblins is just really small, and wearing the skins of people.
You give me the temptation to stat up a minigoblin swarm monster that can collectively hold and use up to Large weapons. They can also wear the armor of their kills, and attack after the players bring down a party of hobgoblins or gnolls or something else with some gear, so they come in armed and armored.
"You see what appears to be moving moss gathering around the bodies of the slain. Suddenly they rise from the dead, or so it seems. Every one of the bodies is swarming with bug-sized goblins, armed with tiny knives. With a collective, high pitched, "heave-- ho!" they clumsily pull back the arm of the gnoll pack leader and swing it forward at the nearest member of the party."
When the bodies are minced beyond usefulness the party can attack the goblin swarm itself, which retreats into the breastplates and helmets and uses the flails like tank turrets. These microfortresses are knee height and absolutely terrifying because they don't go down until someone finds a lot of fire to throw at them.
All the fun treasure is, of course, gone. While you were fighting the horde, their worker caste dragged all the real goodies off to its nest. You're left with the busted up gnoll armor, the flails, a couple thousand toothpick-sized shortswords and a bunch of smouldering leather armor sized for pixies.