great post alex,
here is some info borrowed from another site that I think explains it well, as MIE says dressage comes into play with everything we do (hopefully) including jumping.:
Roundness comes from strong, supple muscles - especially the back muscles. This can only be achieved if the horse's nuchal (poll to wither)and supraspinous (wither to croup) ligaments are fully engaged. These ligaments are the supports of the spine and enable it to carry its own body mass and that of the rider with minimal muscle power. This allows the back muscles (which are not meant to load bear) to work correctly. True roundness can only be achieved if the horse has a balanced skeleton, supple muscles and is happy in its contact. Without this it cannot be forward and straight and without being forward and straight it cannot be truly round. Soft back and quarter muscles allow the hind limbs to step under and to push through; relaxed neck and shoulder muscles allow the fore limbs to reach the maximum length of stride the horse's conformation allows.
Roundness cannot be achieved by pulling the head into an outline - in fact that works against it.
Some gentle suppling work in walk helps in warming up - eg asking for the inside hind to cross over in a small circle, shoulder fore etc. This can also usefully be done from the ground.
Aim for a long low outline, forget all about where the head is, get the horse moving foward rhythmically and straight; get off its back into a 2 point seat and just work on achieving a relaxed and easy trot. Once you've got it, ask for the same in canter; if the horse hollows, go back to trot or walk and start over.
As the horse gains confidence in your hand and your ability to stay in balance with him, he'll develop a bigger trot and once his muscles start to strenghten he'll come up naturally. BUT as you start to take up a contact it must always be soft, consistent and elastic. Your job is to organise, facilitate, balance, show the way - not to pull, hold or dominate.
It's not a quick fix. But when I am schooling a novice dressage horse I aim for a relaxed, balanced, confident horse with smooth flowing paces - I'm not unduly bothered about where its nose is. Hope this helps.