In reply to sharene:
Being about 10 stone and 5'4, I have the same problem (but rarely).
As mentioned above, the frictional losses should even it out a bit... but you can cross the rope over itself once or twice to introduce a little more friction into the equation.
You can also use the sandbags.
There are sometimes bolts at the bottom of a climb (about a foot off the ground) which you can attach yourself to via a sling and krab.
Careful which belay device you choose - I had an ATC which was fine most of the time, but on one occasion when I was belaying someone much bigger than me, it was awfully difficult to hold him when he fell! I really had to hold the rope tight in order to avoid dropping him.