They’re the same species, but it took a long time for that to become common knowledge. When explorers heard ‘Goblin’ and ‘Troll’, they assumed those were names for two species that look very different. Goblins are usually around three feet tall, and gangly with large heads and long limbs. Trolls are hulking monsters, nine feet at the shoulders, and with claws in the few areas that aren’t slabs of muscle.
When someone actually bothered to ask them (and pay for accurate information), they discovered the words were better translated to ‘man’ and ‘woman’. And then probably heard in far too much detail about how that worked in practice.
The story goes: Greenskin spogs grow up in a big stinking pit, somewhere between a nest , a nursery, and a guantlet – there’s about ten Goblins for every Troll. Goblins grow into scavengers possessed of the lowest cunning, who donate food and resources (and, in modern times, guidance) to their Troll, who acts as the sole defender for the nest. When a Troll gets big enough to get noticed by her mother, she gets pelted (with Goblins, the most convenient ammunition) until she goes away. The ‘ammunition’ follows her, steals a load of food for her, and the whole mess starts over again.
Goblins have taken to the scheming and politics of modern life like a Duck to Being Called A Duck. Trolls… not so much. Trolls are supremely suited to ‘ripping intruders limb from limb’, and struggle to grasp that city living might need some other approach. As a result (of some very messy ‘accidents’), Greenskin pits tend to get pushed to the edges of the city, and have congealed into slums.
In terms of the arena, Greenskins are absolute opposites who work well as a team: Trolls are foaming beasts of unstoppable death, but they rely on the fragile and vulnerable Goblins to point them at the right target.
Alone, Goblins rely on alchemy, bribes, and blatant fanservice to get anywhere, assured that the greentops will give them a good writeup after a predictably brief stardom.
Trolls by themselves are very entertaining, in the same way as nitroglycerine waterbombs are.