OK, so I get that there are a few species of large spiders that eat birds or something, so in that case you’d need enough venom to take down something as big or bigger than you are. But what about spiders like black widows? They eat flies, which as far as I know don’t take a whole lot of venom to kill.
And it’s not like the severe amount of danger is a deterrent to other species, is it? Humans can recognize the small markings if examined, but I doubt that whatever eats spiders (frogs?), or accidentally tramples through a spider’s environment (dogs, cows, etc) notices something like that. They’re not brightly-colored like monarch butterflies or poisonous frogs, and don’t have a warning sound like rattlers, so why was such a huge amount of venom a benefit that was selected for? I can’t imagine that black widow ancestors that were taking down flies with half of the venom were actually doing so much worse than the ones that survived on to today.
As you’ll note, my knowledge of spiders and evolution is shaky at best, so please help fight my ignorance on any facts I got wrong up there.