All scorpions have venom; let's just get that out of the way. Their venom is not a defensive mechanism (meaning, they didn't evolve it to protect themselves from us), it's how they catch their prey. This means their venom is adapted to neutralizing the things they normally eat -- namely, invertebrates. So first off, a scorpion's venom isn't really tailored to affecting humans.

Very small scorpions such as the bark scorpion found in Southern Florida can sting and the sting is painful -- but, like a bee sting, goes away with little more than localized swelling. The issue with very small scorpions is they literally cannot inject enough venom to affect a large human. Think of dropping a single drop of red food dye into a big bucket. There's so much water you won't even see the color.

On the flip side, very big scorpions tend to rely on their pincers more than their venom to neutralize prey as pincers are not as costly as venom and don't require them to constantly replace them. Therefore, their venom is weaker and not nearly as potent.

The best predictor of how strong a scorpion's venom is not the size of the scorpion but rather the size of their pincers in relation to their body. An emporer scorpion, for example, is huge, but has huge bulky pincers and very, very weak venom. Another scorpion with more slender, delicate pincers is not using their pincers to catch their prey and will have a more powerful venom.