She's a kryptonian. She has all of Superman's powers, and should be every bit as strong, durable, and difficult to knock out. And yet she was defeated by a single blast while Superman took several and still managed to win. Why? I understand there might be a difference in skill, but not in durability.
- It's never made clear how Kryptonian super powers actually function and given how poorly Supes seems to do whenever he runs up against someone with martial arts training from Krypton it seems plausible that their strength is some kind of multiplier, not an add-on. Meaning Superman is 1000x as strong as an adult male with his body structure is and Power Girl is 1000x as strong as a woman with her body structure is on average. She doesn't have as much durability training as Superman, and she hasn't had to deal with the effects of kryptonite radiation quite as much, so of course Superman's better equipped to handle Lex. It's worth noting that according to official cannon Wonder Woman is the strongest female character in the DCU: and while it varies from time to time Wonder Woman is rarely depicted as being on par with Supes himself, and Power Girl and Super Girl are beneath her.