Windows7 + Office 07

How does one replace a current pst file with a previously dated .bak file?

[Per Google searches this regard, the answer is to just change the filename extension of the .bak file to .pst and use that file as the new .pst file. However, the filesize of the .bak file (dated within several days of the .pst file) is significantly less than that of the .pst file. I then presumed that the .bak file had to to be either compressed or missing something requiring an intermediary step?]

Thanks,

Mark