A Horse&Rider answers:
Stirrup length for any discipline is a personal choice but, generally, you’d ride with shorter stirrups for jumping than when riding on the flat. This is because you need to be able to move your body out of the saddle to fold over fences and stay in balance in a light seat, which requires more angle in your ankle, knee and hip joints to enable this increased range of movement.
If your stirrups are too long, you’re more likely to lose balance, causing your lower leg to swing back and upper body to tip forward, putting you in a less stable position for jumping, and negatively impacting your horse’s balance, too.