Newbie query: why required weight?

["Mary K. Krauss" <kraussberner@worldnet.att.net>]

Is the weight requirement = to rider's weight + horse's weight?(I can just
see 'em lugging up a scale big enough to weigh a horse....)  Or is there a
minimum weight every horse has to carry?  If so, what is it?  and, why is
it? Thanks--email privately if this question's been asked and answered
before. 
mkk