Moehat: I've just trawled through a huge download of the FEI's rules and couldn't find a word on min or max weights, but from the 1996 Olympics, they ruled that the weight would be 154 lbs, or 11 stone, or 70 kg, whichever you prefer! So a short stubby rider versus a long thin one like Ben Maher today could do around the same weight. But that's for Olympic level.
The answer from me is that I don't know what applies, or even if any rule does, for non-Olympic jumping and other equine sports recognised by the FEI (Federation Equestre Internationale, the body governing rules and regs for show events). If you go to
http://www.fei.org, you'll find all the disciplines they govern, including jumping, dressage, driving, etc.
There would be a perceived advantage to riding as light as possible, but, on the other hand, showjumpers are often hefty animals - Oldenburgs around 17 hh, Holsteins well over 16 hh, for example - and as you saw from some of the quite fierce bitting and restraint tack applied, they can take a lot of managing. So being light could have some disadvantages. A showjumping saddle can weigh up to 14 lbs or one stone, depending on materials used, and if you were a bit worried about being overweight, there are always little tricks like using lightweight jackets and thinner boots, rather like jockeys.
It's interesting to note that the Olympics restricts horses to a start age of 9, when they are officially 'aged' (at 8), so pushing horses too early in life is discouraged.
I love watching jumping and dressage. The big chestnut manoeuvred himself very well, although came off-centre a few times at the canter pirouette - a difficult move for such a big horse to make. His extended trot was light and airy, though, with no harshness to it. Simon, dressage is probably the most difficult of equine disciplines in that it requires the horse and rider to be in total harmony throughout. There is never a moment when you are applying anything stronger than signals with your hands, legs, and the movement of your body weight, to indicate what result you're looking for. Basic, simple dressage movements are a wonderful workout for any horse at any age, though - there are lots of movements which create suppleness (rather like yoga for humans), and, particularly for racehorses, help to counteract getting too one-sided due to mostly working their left side on British tracks. Many ex-racers are very stiff at the canter when asked to use the right-side leg to lead, often unbalanced even at the trot when a rider does what is called changing diagonals - the rider does a double bump in the saddle and rises on the opposite lead leg. This exercise provides balancing for the horse, especially those which have been ridden almost exclusively one-sidedly. Yes, there's a helluva lot more to dressage - it's hours and hours of painstaking gentle 'asking' and encouraging, not getting out 'the persuader' to ask for a greater effort.