Whatsup with the massive size fluxes?

To explain the title; heres what I noticed today.

I was opening two old projects, f8 format … editing and resaving as cs3.

Nothing interesting happened there, the filesizes of the 2 files changed a bit, but not considerably.

However, when I saved them as f8 from cs this happened

File1 decreased from 8.5MB to 2.5MB
File2 decreased from 13MB to 7MB

Eeeeep? What just happened? : )