Actual PST vs Folder Size difference

[cybpsych]

hi guys, i'm curious on the size difference (after compact) between the actual Outlook PST file vs the Folder Size (Properties | Folder Size...)

Here's what bothers me:

Actual PST size: 153 MB (160,769,024 bytes)

(inside Outlook) PST Folder Size: 125,685 KB

any ideas why the big difference (153 vs 125)?