by Guillermo Madero S..
Hi Stefanos,
Check the size of those folders not named with a number (e.g. sessions, temp).
The main factor to consider is that Moodle 2 uses a different method to handle files and so it avoids having duplicate files. If you had lots of duplicate files then you should certainly notice a size reduction of your moodledata directory.
Anyway, just make sure you have a complete (and tested) backup of your 1.9 installation.