hello
by default discover incident does not contains original file so they are very light (if you have added a response rule to copy attachment there size will be bigger).
you could not set a timer to send incident from discover to enforce, if you really need incident to be kept at discover level, you could try to block communication between enforce and discover (or shut down your enforce if possible) but dont sure it is a good idea. Is there any specific reason for you to want to keep incident on discover server ?
regards