system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are many Australian spiders, such as the funnel web spider, toxic enough to drop a horse, but prey on small insects?

As Bill Brison put it, "This appears to be the most literal case of overkill".
gpt
Here is the low-down.  Spiders' prey is often much more mobile than the spider is.  Flying insects, jumping insects, and other quick moving prey only need a second to escape a few feet out of the spider's grasp.  Even if the venom kills the prey after it escapes, it doesn't get the spider a meal and is thus ineffective.  This situation means there is a large evolutionary incentive for the spider's venom to be EXTREMELY fast acting, which means the venom is extremely potent.  When it bites a horse, it doesn't kill the horse immediately, but the potent venom still has enough oomph to kill it slowly.

For a similar example in the marine environment, see the cone snail and its gnasty cocktail of toxins!