Hiberfil.sys is what the system uses to store the active memory when it goes into hibernate mode. Pagefile.sys is the operating system's memory swap file. You *never* need to back those up.

If those files exist on a disk drive that isn't actually hosting those files for an active operating system (for example, your live operating system is on C: and your backup drive is D:, and you're seeing those files on D and you didn't specifically tell the OS to put them there), then they can be safely deleted.

Regarding file size discrepancy: As was stated elsewhere, the size of the allocation units on each disk drive is a factor. Imagine a disk drive as a large grid of allocation units. Each grid might be 512 bytes each, or 1024 bytes each, or 4096 bytes each, etc., depending on the size of the drive, the filesystem, and how it's formatted. A file fills up a bunch of allocation units and then the remainder (if the file doesn't fill that last grid completely) is wasted space. See Cluster.