Why are the PAGE_SIZE and the physical page frame size different in x86_64 kernel?

Hi, I'm confused about the meaning of the PAGE_SIZE macro in x86_64 kernel. Its value is 4KB and I once thought it was the size of a physical page frame, but now I think I'm wrong. I find that most of the physical address space is mapped by PMD entries, which means the actual physical page frame size is 2MB. Could someone explain why the PAGE_SIZE is not 2MB in x86_64 kernel? Thanks! Regards, Hao Lee