Making fight decisions based on "could I kill it" is a convenience of human technology. The ability to seclude ourselves during healing and medicine allowing us to avoid infection, heal faster, and heal from more serious wounds has skewed how we think about fighting. Most animals make fight decisions less on "can I kill it" and more on "how badly can it injure me".

Sure a human can kill a house cat, absent technology can the human do it without having the skin on an arm or leg shredded? Will the injuries be significant enough to make you unable to protect yourself from other predators? Will the injuries set up infection and kill you?

Cats are basically the perfect land predators. Even with their small size domestic cats are the most deadly and destructive hunters on earth.

https://www.scientificamerican.com/article/cats-kill-a-staggering-number-of-species-across-the-world/

They are ambush predators. They are really good at evaluating prey, identifying strengths and weaknesses, figuring out how, when, and if they should attack. Cats know whether or not they can win a fight. Cats will sometimes charge into fights they can't win, like attacking the bear, because they know that they can inflict damage and that the other animal is making a similar fight decision. The hyper aggression of a 10lb claw tornado flying toward a 200lb bear is usually enough to convince the bear that the fight isn't worth it.

I don't think that they are too aware of size differences, based on plenty videos that I've seen of cats chasing bears away. Here's one of them.

Instead I guess that what triggers them to think "fuck, it could kill me!" is 99% behaviour.

I've always felt that we are like clumsy giants to them lumbering on throughout the home with the great strength and dexterity to manipulate objects. In spite of all of this they can swing from loving you to down right cold shoulders on a whim!

My obligatory cat tax. :)

To be honest, I don't think your cat thinks about things the way you're thinking about them. Your behavior has shown your cat that you're safe, so your cat isn't afraid of you. If you start acting aggressive or scary it will treat you like a threat and fight or flight based on what it decides in the moment, but that's about it. It's not thinking, "this giant ape could kill me if it wanted, I'm glad it's my friend," it just has a positive association with you based on your pattern of behavior.

Also, cats social cues aren't as obvious as dogs, but if you don't know what you're cats thinking, here are some clues: is your cat walking around with its tail up a lot, usually curled at the top like a little question mark? Your cat feels super happy and confident in it's territory (AKA your house). Does it ever look at you and narrow or close it's eyes and look away, sorta like it's nodding off? Your cat is saying it trusts you enough to let it's gaurd down, it's basically a cat hug.

Edit: cat tax.

When cats square up to each other they arch their backs and their hair stands on end, to me, that's an effort to make themselves look bigger. Therefore they know bigger == more dangerous.

Cat tax:

They use the same communicative signals with us as with their young. To cats, we are overgrown, clumsy kittens that never learn to hunt but know a mysterious way to get food.

Cats don’t have to worry about size because they already know that they have all the power in the relationship.

My Izzy (cat tax:)

Isn't even slightly afraid of me; I'm her favorite sleeping surface, she doesn't hesitate to climb up on me, step on the most sensitive giblets she can reach, curl up and take a nap. I'm pretty sure she does know how much bigger I am than her, but she doesn't seem to let it bother her. She's not even afraid of getting stepped on, judging by how much she likes being underfoot.

I remember reading on the old site that cats saw us humans as "big, neutral cats".

Cats can be pretty enigmatic and have a lot of individual differences in personality. At least size can't be the whole picture, based on tiny tiny kittens squeaking terror into the hearts of giant four-ton beastly hounds, bears, what have you.

Maybe we humans are just size queens, as nothing stops a chihuahua or something from doing the same.

Some people have suggested that cats get to see you as a "pack member" - dead birds and mice and stuff are gifts because you're just too incompetent at this hunting stuff, so they graciously help you out.

They would notice if they cared. They do not.

My wife and I adopted a kitten when we got engaged. And up until we had a baby a few years later, that cat 100% thought it was going to grow up to be like us.

I always think of my cat as a lovable idiot little brother. Sometimes he wants to be like me. Sometimes he thinks I'm a huge idiot and wants to go his own way.

Yes: https://www.cnet.com/culture/scientist-cats-think-you-are-just-a-big-stupid-cat/

My cat makes up for it by sitting on my shoulders. I specifically have several canvas or thick jackets I wear around the house to protect me from his claws (we do trim them) because he just hops on me like a Taxi.

I am sure they are aware of it. I think it's like us and horses: yes, this creature is much larger and could hurt or kill me if it wanted to, but horses are basically cool and friendly and we trust them.
I've been with my SO for more than 4 years, and I often don't understand her motivations either.