Quick question about evolution

[Mermaid]

I think some specific prey animals had evolved increasing immunity to the venom, which led to venom arms races causing some snakes to be incredibly poisonous to other animals that aren't normally prey

I've read about arms races, didn't think of that though, you're porbably right. 
Some thoughts: 

Because the prey is usually very small, the amount of venom they get is a lot relative to their size, so even though they are better adapted to handle it, they react just as strongly to the venom as we do...?

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Btw, will make an intro!

Hi there. Evenomation is not a constant; sometimes there will be a lot, sometimes, not very much, and other times, not enough. It's a trade-off. It can range from about a 1 mg to 1 gram for a rattlesnake, for instance, depending on several factors--age, size, body condition, time since last bite, etc.
Venom is a super interesting thing to study. There are some venoms from spiders and snakes that contain proteins with a whole spectrum of activity. If you fractionate it, you can get venom specific to insects, or mammals, or birds, etc. Venoms can contain proteins that specifically target blood clotting, or some nerve impulse transmission mechanism, or several other things. Some animals have more of a certain mechanism to interfere with than others. What kills a person may only cause a dog momentary paralysis. Black widow and brown recluse spiders, for instance, typically aren't as dangerous in a dog as they are in a person.