Q: If my cat, who is a gentle house cat, suddenly became as big as a lion, would he kill me?
A: That depends on a lot of factors like age, activity level, if he’s neutered, how he plays with you, his personality, and your bond.
If he’s young, and very exuberant about play, and is serious about his aggression toward toys, then it’s a possibility. If he was taught (or not corrected—gently and positively—when he has attacked your leg or something) when young that human hands or other body parts are toys, then the answer is almost certainly yes.
I actually use this example as a thought experiment a lot when I’m talking about why big cats do not make good pets. Think about a young cat, maybe a year old. At that age, you can see how intent the kitty is about attacking and “killing” a toy, which is a thing it just perceives as being something fun to maul, not even real prey. Then imagine that he was the size of a medium-sized dog, and ask yourself what kinds of things he might think look like fun things to attack. A little kid would be fun sized then. Then imagine the kitty is the size of a tiger, and you should get it right away. It would be a disaster for everyone.
Make no mistake, there are fundamental differences (at the genetic level) between domestic cats and wild cats (big and smallish). Domestic cats have changes on genes dealing with aggression and learning. They are less aggressive and are able to learn more and learn faster than their wild cousins.
However, older cats, especially neutered cats over the age of 7 or 8, wouldn’t necessarily pose a deadly risk if you magically scaled them up. By that time, they don’t see you as a plaything (again, unless you have encouraged them to play with your hand or not corrected them—gently and positively—when they have attacked you), and are usually more interested in naps, food, watching Cat TV (AKA the window), and other things.