I've noiticed that some game installers have other sizes now comparing to when I downloaded them first.

Examples are :

setup_freespace.exe : 830 mb => 808 mb

setup_freespace_2.exe : 1.55 gb => 1.53 gb


This difference in size is this because of game updates or Win 7 compability ?

updates

Very likely to be updates. For example:

Freepsace was last updated 2010-09-22
Freespace 2 was last updated 2010-09-22

http://pastebin.com/mwJQ8XdV