I think it is important to ensure that you are actually comparing like for like, if comparisons are necessary, and not 'apples against oranges'.  
A 'normal' installation of Windows XP is basically just the operating system and a few tools and utilities to help use it in some basic ways.  Yeah, it takes around a Gig or so but you are than going to want to install applications to be doing much usefull with it - and the way in which applications install under Windows impacts both the available space and the performance of the system badly.
A 'bare bones' Ubuntu instalation will be somewhat smaller than that, depending upon just how far you want to go with optimising it.  By default, Ubuntu will let you install a whole load of useful applications at the same time as you install the op system which of course will take up more space, but not really impact the prformance of the system.   
With storage space being so cheap these days, even 16Gb USB memory sticks cost hardly anything, most people make their choices based around functionality rather than size.   Basically you one choice if you want to play games and experience all of the latest virus and spyware threats, and another choice if you are on a restricted budget but want a secure and efficient system to really get things done on.   
Like the motor manufacturers omit to say:   'You mileage might vary'.      

Kevin Lawton

(0161 654 8944 & 07973 303785
*kepla@btinternet.com & kepla56@hotmail.com 

From: Niels Egberts <niels.egberts@gmail.com>
To: ReactOS General List <ros-general@reactos.org>
Sent: Tuesday, 29 September, 2009 7:50:23 PM
Subject: Re: [ros-general] Keep it Clean

On Tue, Sep 29, 2009 at 3:40 PM, Bernard <bernarddt@gmail.com> wrote:

When you install a new OS the size of the installation astonish me everytime.  I looked at Ubuntu that seems to be so nice according to everybody, but it uses almost 10Gig after installation.

Not that it tackles your piont but Ubuntu uses waaay less than 10GB. It comes on a 700mb disk. The requirements are:
At least 4 GB of disk space (for full installation and swap space)

I think the default installation is below 3Gb I think.

Niels.