The Mezzanine

I've always thought that Slayers were stronger than vampires (and Angel outright says it), where as vampires are faster than Slayers and more aerodynamically gifted (like how Angel demonstrates his climbing abilities in "Habeas Corpus").

I think she is significantly stronger than a vampire, but not to an extreme degree. And I think there's something interesting in the idea that the object is mystical and that's why she's more suited for wielding it. There's also the idea that Buffy wanted to lift it more, she was more determined to carry it, and that factors in.