system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do venomous animals have the potential in killing humans yet only eat small animals?
gpt
Most them have venom because they need their prey to stop struggling immediately, maybe it's because they are slow and they need their prey to stop running before it gets far, or maybe because it wants it's prey to die before it can fight back.

In both these cases they need venom that is strong enough to kill in seconds, minutes or hours is far too long and wouldn't be anywhere near as useful. A good example is the cone snail, it throws poison darts at fish, it needs to kill the fish so fast that it can't outswim a snail. It's venom is that strong, and it does stop a fish from swimming in about a second.

Anyways, venom that can kill a fish or mouse in a second tends to be quite strong, and it's still deadly to things much much larger, even if it takes hours or days to kill them.