But I do not concede that poison lethality need coincide with the size of the creature. I'm thinking mainly save DC. That doesn't sit well.
Of course it doesn't have to, but it should be the common situation. It's true that there are spiders and snakes that are potetially lethal. But their numbers are small compared to those that are just painful/distraction to humans.
And yes a small, but infamous unimaginable poisoneous creature, can be a fun thing in the game. Especially with forewarning