Why does it need that much hard disk space?

[Cris Ali]

Hi Folks,
I was surprised to learn that installing linux requires more than three gigs
of hard disk space, 20 times more than windows98.  If i am to install it on
my retired 90 mhz pentium, i have to buy a new hard disk.  This machine has
two hard disks, one is 2.2 gigs, and the second is only 320 megs.  Is it
possible to have a small installation of linux, at least to learn how to use
this system?  Secondly, does anyone know a good place to buy inexpensive
hard drives that would be competibile with old computers?
Cheers,
Cris Ali